Ah, that's just a straight up bug then if it comes out of your pocket instead of Shadowheart's pocket with Therezzyn. I personally wouldn't call it weak writing, just the wrong animation playing. The animation for Shadowheart is definitely available and you could probably even trigger the conversation keeping your PC character separated somewhere else and have whoever you choose to speak with Therezzyn with have it in their pocket in that case rather than the one who actually has it in their inventory. That's a bug that should have been squashed ages ago in that case, but the people who encountered it either didn't report it or they never handled it.

I saw something half-bugged in the Mindflayer Colony with the brains in jars and the elven head you can talk with. You can talk to them with Jaheira and she can have telepathic tadpole-like connections for some weird reason, but then finally when the Githzerai offers to transfer the mind barrier buff you do only get to say that you don't actually have a tadpole when talking as Jaheira instead of your PC. Weird inconsistencies here and there. You can also trigger unique conversations/cutscenes sometimes for whatever reason, like if you're on a Tav/Durge playthrough and have the talk with Nightsong in camp about the real wolf memory but you choose to do it with Shadowheart and your PC is on the other the other side of camp, you'll have a different variation of the talk between Shadowheart and Nightsong without your PC involved and without dialogue options in-between. Same as when you stay back in the House of Grief boss room on a Tav/Durge playthrough and have Shadowheart approach her parents entirely by herself, she'll have a unique cutscene/conversation without dialogue choices that's also different from a Shadowheart Origin playthrough as far as I know.

When it comes to reporting bugs like you had with Therezzyn though, my personal experience is that any inconsistency or bug I report, even going back a year ago, unfortunately never gets handled or acknowledged. So I imagine people have noticed it like you and have already reported it, but for whatever reason Larian never did anything with it. I made a bug report and thread about Halsin's completely broken romance and consent flags almost exactly a year ago where people massively agreed it had to be looked at and fixed, and in there I referred to a similar popular thread from a year before that. Despite the majority agreeing that Halsin's romance and consent dialogue flags are completely broken at certain spots, they haven't looked at it and probably never will. Thread's here. And there's also a bug that they introduced in January's patch 8 stress test that is still there unfortunately, one I mentioned here and you can find video examples all over Reddit.

So yeah, personally I'd only consider certain moments in act 3 as weak writing, other things here and there I'd consider unpolished, unaccounted for variations and/or unsquashed bugs that haven't been looked at and fixed. A funny variation which they do account for in the creche is when you disguise every companion as Gith btw, they treat you far more accepting in that case and even with the Vlaakith projection they'll allow you to pretend to be a Gith with certain dialogue choices.