Scenes that contradict previous events (like, for example, Ascended Astarion breaks up with Mindflayer!Tav then 6 months later it's like nothing happened, they are still together). Weird gameplay things, like, for example, Shadowheart's both good and evil paths having the exact same approvals with no difference at all. Stuff like that.
This one is due to a problem with flags and a problem with just... how rushed his specific epilogue is. I don't know if that's the case for other romanced companions, but it barely takes a few choices into account, all across the board. The flags that it sets for the situation are set up for a reason but then not brought up at the epilogue. There are more contradictions at play there, but they're contained within the devnotes, so I guess that's dubiously canon.
I'm not savvy enough about game development, but I'm not sure other games have such big issues with the flags. Watching the SlimX videos about how much content is broken is staggering, and the worst part is I'm not sure the devs are aware that it's not working because no one was able to tell until now that that content wasn't triggering.
