If you know what a rest, shot or hold is as known to BIAB/RB any of those will cause a gap in generation as will having the 'two bar ending' checked. Both of those conditions caused gaps in generating the tracks in some of my testing. In the RealTracks Style test, this occurred at bar 16 and again at bar 273 because I had input a hold ( G... ) at bar 16 and RB automatically generated the 2 bar ending after what I designated as the last bar of my song.

You can test your system by replicating my test. Create a new file, be sure there's a BIAB style picked, (this should be done default) and just any a chord every 1-15 bars and set the length of your song somewhere above the 255 bar. If you enter a rest, shot or hold, you must enter another chord after that after a number of bars to cancel the rest, shot or hold command. The test should separate your issue between a system or program setting and your imported file.
BiaB Ultra Pak+ 2020:RB 2020 Build 1. Dell Inspiron AIO Desktop; Windows-10-64 bit, 7th Generation AMD A9-9425 Processor Five Core 3100 Mhz and 16GB DDR4 Memory.