Every time I exit the lobby I lose almost all gained experience and loot

I played with my friend for 2 weeks, and every time after the end of the session I lose most of the experience and loot (including Guardian Ranks).
Previously, this was not so problematic, because we just played an extra 30 minutes and were happy, BUT the last time we went up to lvl 50, processed a lot of high-level loot and ended the session. Today we started another session, and I became lvl 48 and lost all my lvl 50 loot.
I don’t know why this happens again and again, we tried a different order of exiting the game, etc.
Can you explain how to avoid this? This is the disaster.

It only happened to me if the host quits the game.

Try to quit before the host ends the session. At least it worked for me.