With the late addition of Halsin as a companion and poly romances becoming a thing, they didn't have proper time to implement it consistently throughout the story unfortunately. If your Tav/Durge is attracted to him and like the idea of a poly romance with your other established romance partner, it's a nice addition to experience that. Personally I feel in this particular case poly romances aren't implemented well unfortunately, precisely for the reason you are showing with the sudden shift in character without inbetween on-screen character development portraying this change and also a severe lack of communication with the established romance partner on how you view things together.
To be able to add in Halsin's romance and particularly poly options so late into development, to me it seems that too much surrounding Halsin's romance and other companions behavior towards poly romances had to be turned on its head at too short notice. I think it would have been a lot healthier of an approach to have the start of a potential poly romance and/or open relationship beginning with discussing the option with the established romance partner. Not just a few lines, but a full dialogue on how both sides look at things, what they are or are not okay with, why they view things a certain way... these kinds of things. If you could start the conversation with them first and come to a healthy balance with enthusiastic consent, boundaries, trust and openness first, only then you can begin with outside influences as potentially adding other people into the mix. Starting the path towards a poly and/or open relationship with "hey that person that's been travelling with us wants to be involved with me romantically/sexually" to me is a very awkward and unhealthy way to begin a conversation on potentially changing the established relationship.
And this being a video game, player agency in these things is extremely important because for a lot of people these roleplaying experiences are a way to escape all the messy and confusing real life interpersonal connections and take control over your own fictional story so you can really get immersed into it and attached. To have that feeling for two full acts, to experience a carefully crafted story and development of characters individually and relations with each other, only for them to then get a whiplash of sorts because of a late addition of something (in this case poly romance options for Halsin) that isn't as thoroughly written and developed as what came before, is understandably disappointing and frustrating for people.
If you're already acquainted with poly romances and/or open relationships, it's easier to look past the sudden jank of a tacked-on new companion romance with poly options and head canon how an established romance partner might off-screen work their way towards a shift that's now written for Shadowheart's views between act 2 and act 3 for example. For it to be a healthy development and portrayal of a poly romance, it should still definitely start with a conversation with the romanced partner first and working everything out together before even entertaining the idea of introducing someone else into the relationship, but with the way that's it's written now, it's clear to me they rushed past that to get to the "juicy bits" of characters banging each other.
If you're not acquainted with poly romances and/or open relationships though, or if you're personally against your Tav/Durge being involved in anything other than a monogamous relationship, to me the current way these romance/sex options are hastily written are very damaging for the established view on the romance path you've built up to that point. Because to me this late addition and option feels so tacked-on, it's a horrible introduction to something beyond monogamous relationships. Bluntly put, it's a dude barreling into your established relationship and inviting himself where he is not (yet) welcome. If you then tentatively broach the subject with your established romantic partner, not even because it's something you want to do but because you're confused by what the third party just suggested, your partner's first response is to enthusiastically go for it without them even considering what your own views actually are. To me, with the way it's currently written, a lot of player agency and consent is out of the picture and all parties are super excited before you've even had a chance to process what was even really suggested and that adds a lot of pressure in what is supposed to be a safe enviroment within your Tav/Durge's relationship.
Then of course things unfortunately go wrong even further because of dialogue flags not properly setting and Halsin seemingly not taking no for an answer if you reject him and him propositioning for sex a few times after desipite your lack of consent, either specifically with you or with you and your romantic partner. Logically you can see this sequences of interactions as (what I hope are) dialogue bugs not flagging the rejection properly, but in the moment in-game your Tav/Durge now has a creep trying to force himself on them and their partner is seemingly okay with that.
In my opinion, there needs to be a lot more care when it comes to approaching a subject of relationships other than "standard" monogamous ones. It's a subject that needs to be approachable and discussable within the game from the outset and at the start of relationships, not tacked-on at the end when you're practically married with the person already. The subject should also be introduced to the player through dialogue options with the romantic partner, rather than a third party suggesting the option. And there should be player agency in a video game in how to approach it and what their decision is, rather than the established romance partner taking that agency away and telling their views before you've had the chance share yours. And only once a baseline and balance of enthusiastic consent, boundaries and trust is reached within your established romantic relationship, only then should other parties be thrown into the mix. Because the writing regarding poly romances and open relationship did not get the necessary attention and care it should have for such a complex subject, it currently strongly pushes people away that are new to these topics. And therefore I find it very unsurprising that it alienates people from these concepts, that it sours their platonic relationship with Halsin in current and future playthroughs, and to some extent also damages their established romantic relationship in that playthrough because in some aspects the established partner suddenly seems like a stranger to them compared to how their on-screen character development was going in the first two acts.
