Many thanks to Ulodesk and Stefan Blom for their responses.

After "sleuthing", I've found out what triggers the re-appearance of the formatting bubbles.

I constantly use the On/Off for "Track Changes" in the status bar. One reason: Word leaves no space between a deleted word and a substituted word. I believe this makes it more difficult for reviewers to read the text. There are other reasons related to legibility.

(Note that I don't blindly accept all changes. I accept/reject one at a time, so I can see any extra spaces - or do a global search for multiple spaces.)

Here's the sequence.

- Select Off in status bar.
- Deselect "Formatting" in "Show Markup". Formatting bubbles disappear.
- Change Off to On in status bar.

This always causes (a) the formatting bubbles to re-appear, and (b) Formatting changes to selected in Show Markup.