Hi,
I was recently looking into this same behavior while playing the game for the first time, and I thought I would share what I discovered here in the most relevant existing thread. There are also some other online discussions that mentioned how experience gain would be greatly diminished for characters further down in the list of inactive party members. I was looking at the Assembly-CSharp.dll file, according to another post I had found, and I noticed what I believe is a minor error in the code. It seems the game is intended to give inactive party members 90% experience if they are equal to or below the player's level, and 50% if they are above (based on global.gamedatabundle file). However, these adjustments are made to all of the characters in a loop without resetting the initial experience value on each iteration. Therefore, the first inactive member would get 90% xp as intended, the second would only get 90% of 90%, or about 81%, the third would get about 73%, and so on. Depending on the number of inactive members, this can result in severely reduced experience gains for the most recent members not in the active party. The post that shows this code and how to edit it is below. The poster just bypasses the loop and assigns 100% xp every time, but it would be easy to just add a new variable for reassigning a new adjusted xp value on each loop to make the code work as intended. It seems Obsidian is finished making minor patches like this one to the game, but it seems to be a simple fix in case anyone is interested.
