BG3 doesn't split XP, you're on the same levelling curve whether solo or with a party of six and XP is synced such that any party member with less XP than your main character will be brought up to match them. Note this doesn't work in the reverse, so it's possible to get companions that are higher level than the player character (though this is only really likely to be seen when speedrunning).
For the most part, it also attempts to prevent double-dipping on XP, in that if you kill an enemy after talking your way past them, they won't award any kill XP.
Yeah, this catches out a few people and is a bit of a silly outcome. I think it's an engine limitation - enemies can't go through zone transitions so the ones outside can never reinforce the ones inside and vice versa. That limitation also means the optimal solution is to fast travel out from the indoor area so you can engage the outdoor ones from a safer distance. (It's also a bit dumb that two of the leaders can be assassinated properly with the camp remaining neutral, but one just sets an unavoidable faction-wide aggro flag)
Speaking of engine limitations, the text-only "books" in the game are a pretty glaring one. The system doesn't support graphics so even if you're looking at a map or a drawing, you just get text saying stuff like "this is a map showing an X nearby". The D:OS engine has done good service over the years and has seen some nice upgrades, but it's time for Larian to move on.