Headings and numbering glitch(es) in MS Word for Mac 2015 Version 15.18
Finally I found an answer to this question. It appears to affect several versions of Word, because the fix at MS Answers appears to work on Mac Word as well.

Follow-up question: Anyone know if resetting the normal.dotm file on all PCs that need to access the affected docs would help avoid this in future? It's clearly something to do with invalid combinations of styles, formats etc. and maybe just using 1 common baseline would clarify the root causes?

Successor question [should I have asked this here?]

UPDATE: And... the problem returns upon saving (the corrected version) and reloading.