Art,
Over the years I've noticed some erratic behavior switching back and forth from BIAB generate endings and tags. Similar behavior I never quite figured out.
It didn't seem to happen all the time but it was annoying.

I do remember one fix that sometimes helped.

This may (or may not) work for you.

Check the tag exists box.
Delete all the measures containing chords AFTER where your ending should be.
Make sure there are no chord changes following where you want the song to end. I'm speaking specifically of what will be the greyed out area after you switch back to a biab ending.

Un-check the Tag Box. Re-check the generate ending box.
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biab(Mac) Logic(daw)
Macbookpro(2013)
Purple & Black Express.

Music: https://www.youtube.com/channel/UC4X2ibP-Fz9evpzHiJpAZig/featured