I've noticed that when I take a piano performance and export it to midi and play the file in, say, pianoteq, certain bars are silent. I mean, they play just fine inside BiaB, you can hear the notes playing from there, but the moment it's exported in any form, certain bars just disappear from the file.

I know it's not my computer, because if it was, then this issue would affect bars at random, though these missing bars seem to be the same licks over and over. Is this on purpose? Are some of these licks just, like, piano trade secrets that they don't want to give up? lol

Or is my BiaB just broken? I'm using Win 2021 version UltraPAK with RealBand.

I'd appreciate any light shed about what's going on.

Could this be a RealTracks issue, or are you sure they're really MIDI?
What colour are the track descriptions in the mixer?

The question you pose falls within the category of "BIAB is doing something it should not be doing". Since no one else is reporting this specific problem, the first step to fix this at your end is to hit the "Return to Factory Settings" button. That's the protocol.

Start here: