Vanishing keywords

I'm running Windows 10 with the latest version of Lightroom Classic (12.1). I've been using LR since the very first beta version and currently have a catalog of 236k images that are meticulously keyworded. My keyword list contains thousands of keywords in a multi-level hierarchy.

I spent much of November and December traveling and doing photography. Upon my return I spent hours keywording all of the new photos. Then a day or so later I noticed that all of my keywords for those months were missing. The geotag data was still there though. I then spent hours re-entering them again and thought all was well.

Since then, I've added a number of new photos in January. The other day I noticed that the keywords were missing again for all of November and December, but everything from January was still there. I'm rather baffled by this. I'm going to enter a portion of the data again and try to and figure out what is happening. I figured I should also ask here in case anyone has an idea of what could be the source of my problem.

Thanks!

Erik