I think I just stumbled upon the problem and the solution.

For some strange reason, despite compiling your total available styles in a neat compendium that you can browse through to make your style choice, BIAB fools you!

Yes it actually list styles that YOU DON'T HAVE!
Very strange. Now how am I to know I chose a style that I don't have?
Why was it offered to me in the first place?

Apparently I didn't have 'Bossa New York Piano Trio'.
But BIAB didn't object when I selected it.
I played it, and it sounded pretty decent.

Then the dropouts started happening with the piano.

Then I did a regeneration and I got the famous BIAB error notice in the bottom right hand corner. This one said:

"Cannot generate RealTracks, because this folder is not found
D:\Music\BIAB\RealTracks\Riano,Acoustic,Rhythm BossaNY Ev 140\. RT#1662. Check your Options-Prefs-
RealTracks folder setting, and make sure that you have installed the needed RealTracks"

So plugged in my BIAB install USB stick, found the style and installed it.
Then being a good boy, I refreshed my compiled styles and chose the style again.

This time the correct style played and there were no more piano drop-outs!

It was just BIAB up to its old tricks. smile

A BIAB user for more than 30 years (if you can believe it) !