I had a look at your issue.

As far I can investigate, I have narrowed the issue to the following :
1) tones involved: Ab, A, Bb.
2) tested chords : Ab7, Ab2, Absus - A7, A2, Asus - Bb7, Bb2, Bbsus.
These chords may have or not the Tab displayed on the the notation sheet.
When you regenerate the song , you may get the tab for the Ab7 but nor for the Bb7. The next regenerate time, you may get the Tab for the Bb7 but not for the A7 and so on.
Other chord types may be involved.

See attached view :

I report your issue to the developpers.

701 Bass.jpg

Reach for excellence