Thanks, Peter. I used the 4-button relaunch method followed by the renaming/replacement of the two files, as described in your link. Then I relaunched the software and started a new document. The behavior is still there.
There are several other files related to it, alongside the InDesign SavedData file in that folder (Mac):
Are those also trashable?
Thanks! It looked promising (or I was just hopeful), but unfortunately it didn't fix the problem.