This will seem very strange, but the piano fills (actually midi guitar fills changed by VST to piano) are not completely dropping out, but only 99.98% dropping out. What I am getting is an isolated note here or there from the piano. I am totally mystified. Perhaps that clue might help somebody figure this out.
In the piano roll view check for velocities. If the piano is using velocity for volume that could be the cause, I.e. very low velocities.
If that isn't the case using the MIDI editor check for volume changes. That would be CCs 2, 7, or 11.
What piano VST are you using? Some electric pianos do have have all 88 keys so if the note is off the EP range then you will hear nothing. If you are using a GM piano then forget about this.
Note that you said fills. Normally fills do not play all of the time. Is that the problem?
Let us know what you find out.
PS - An after thought, make sure that all of the fill notes are on the same channel and that there are no program changes in said track. Again using a MIDI editor look for PG. That is the name of program change.