I really appreciate the suggestions everyone offered on this. I did send a log file of the issue to info@... and did not get a response back from them.
In the mean time, I have been trying a number of other possibilities to stabilize NVDA with Outlook and Word, and I think I have isolated the problem: when I set braille to None, the issue goes away!
Unfortunately, I need braille to be productive.
Can anyone suggest additional things to check for in the braille chain that could lead to the instability that I described? I'm using a Focus 40 Gen5 sometimes with USB and sometimes with BT.
Many thanks in advance for your insights!