My problem appears to be solved by setting `comment-auto-fill-only-comments' to nil.
I will grant you that this should not have solved the problem in text-mode, and indeed text-mode does now auto-fill properly without it. I have clear notes that it did not work in text mode, but I can no longer reproduce that even with a restored copy of the init file I was using when the problem appeared. I cannot explain it unless my text-mode tests were inadvertently done in some other mode that has a comment syntax. It is possible (though it seems unlikely to me) that they were done in default-generic-mode as I have experimented with using that as a replacement for text mode in order to use its comment syntax. The time frames don't match up by a long shot, but I may have inadvertently run some vestigial code from another file that effected that change.
At any rate, I think you can safely close this bug.