Sealed system as in no leaks? Yeah, I pressure tested it a few months ago and it was tight as a drum.

Which system are you referring to that would pull vacuum on the tank?

I'll try leaving it uncapped for now and see if it helps. It seems to be more so when it's hotter, or steep inclines, (although it's happened on normal roads after hours of driving) so unfortunately I have no idea when or if it'll happen again. Not something I can replicate easily, but definitely am trying to figure out how to prevent it.