I did that.
The original thumbnail DB was 1,735,552 KB; the new one is 25,884 KB.
I did manage to lose about a fourth of my Album thumbnails in the process.
I sort of had high hopes for this fixing things; but, alas…, the problem remains.
Although this is affecting many hundreds of images and it is hard to see and remember them all, on the tag searches that I do remember, it is always the same thumbnails that have the problem.
And, although I may find out to the contrary, thus far, it only seems to be affecting the tops of the tag hierarchies.
For instance, if I have a tag “cars”, and within that are sub-tags “red car”, “blue car”, and “green car”, with each of those having their own sub-tags.
I can click to show all of the cars and maybe half of them will not display, even though the information in each of the blue album separator bars will display the correct number that should be there.
If I click “cars” and there are seven “red cars” missing, I can click “red cars” and there they will be.
I never realized how much I used the tag search until it started having this problem.