"Characters" inventory keeps disappearing

Using SAB 6.1

The characters lists under our books keep blanking out. They only show properly when SAB is freshly opened. I have not found a trigger, just after some time of working with SAB and coming back to a book and the “Characters” the entire list is emtpy.

When the list is not showing, then the button “Validate Fonts” is giving false results in the sense of “no characters found in this book, so the font can handle that”.

What is your source document in? SFM or Word or …? What script is it?