I was able to correct the issue with cello suddenly--at the same spot in the score---locking XF at a set value. I copied a few bars from an earlier iteration fo the score and pasted--and fixed.
So, it seems to be something in the score itself--invisible as far as notation/markings.
Now it has reoccured at the end of the piece in all strings.
I can likely fix it with the same method, but what could this be?
Looking forward to Dorico.😐