Easily the epilogue is hastily slapped together fanservice and certain things make this *abundantly* obvious. For instance if you choose to travel the world with Astarion (and he cheerfully agrees) you still are throwing parties and there's *zero* dialogue about this. If you are Astarion's spawn and dumped him before the brain hit there's also *no* difference between that or dumping him pre turning into spawn. It's a video game. Some of the writing is going to have holes in it by the medium's very nature because the devs either forgot all variations or didn't accomodate for them.
I fully agree with what you're saying here, especially in regards to Astarion, not so much other companions- that being said, it can also go both ways. I'm not privy to what they actually intended of course, but I feel that way and suspect the same reasons when people ask "then why didn't he compel Astral Plane/Avernus Tav at the party?" It could easily be that they forgot about that possibility or swept it under the rug at the time, and it was easier to write a couple lines acknowledging it when it was noticed patches later (that and/or Larian's philosophy regarding player agency, which I feel sometimes supersedes narrative). It does feel like it's structurally weird, because he talks to you as if another friend unless you bring it up or pick the right dialogue options, so it could fly over the player's head when realistically, it's something (you going away) he would bring up in more depth, regardless of what you think he's capable of doing in regards to compelling Tav.
It's complicated to discuss those, because yes, they are in the game, and thus, canon- but it's not hard to speculate why some things are the way they are, especially when you know of other budget cuts that happened leading to similar things. Of course, if someone says "well, it's in the game" you don't really have an argument against it, just suppositions, because yeah, I mean, that's true. At the end of the day, I don't know why they did it, or if I'm right in my assessment, but eh, I guess I'm free to say what I think : P
