For those following my search problems, I have been methodically going through my extensions. One of my favorite extensions is askvortsov-categories.
I have had the "sense" that my search requests have more latency than before. I have no actual idea what Flarum or its extensions do processing-wise, on their own, in the background (if anything).
However, I noted there is a toggle switch in the settings of the "Categories" extension, and it reads:
Optimize for small forums?
This will give more accurate discussion/post counts, but will slow medium and large forums dramatically.
I turned this switch OFF.
Now when I search, as long as I wait for the items in the quick drop-down box to populate before hitting ENTER, my search seems to be working as before, which is perfectly. But if I don't wait, I still have the issue of getting no results.
This is still different behavior than before 1.8.7, but I can easily educate my users to wait for the drop down before hitting enter.
I'd be interested to know if others with the issue are using the same extension I am and if this solves the issue.