So still happening, wanted to give some more data in case its helpful to the developers:
As Iriarte says if you hit a battlepass level, that amount of XP will never be lost.
I have been able to play a round to get 1/2 way to the next BP level, quit/restart the game immediately and see that the XP is still there.
If 'some' period of time (hours?) goes by before playing again, the next login has the BP XP reset to the amount it was at when you last passed a BP level
All other info for my shelter seems to be persisting fine regardless of time between games...things collected, build progress, food donated...its just the xp going poof.