Thanks for playing and glad you enjoyed it!
Party members left (and you get a replacement at a higher tier) once they were fully upgraded (assuming they weren't at max tier, and a couple extra little checks like that you had enough XP left over to level up at least one of their skills, or if they were way behind the rest of the party's tier), but there was no way you could prevent it (and, mechanically, you probably wouldn't want to, as the higher HP+shield from the tier increase kinda trumps everything else). It understand it didn't really feel good though, as a player =).
Originally I had the party dialog all deactivating after reading it once, but a little too often I ran past one, and then stepped on another, and was like "wait, what was that?", and of course there was no way to go back. I'm not sure if anyone actually went back to read things they kinda missed or not though. Glad to know it spiced up backtracking though! The quantity and quality of those dialogs is all thanks to the writer on the team, they really do add a lot!