I realized that complete branches of my keyword hierarchy disappeared from the catalogue.
I have following keyword hierarchy used for people (for face recognition)
A - [Names starting with A] B - Bill Miller - Bjorn Smith - ... C - [Names starting with C] ...
STEPS to reproduce
1.I have a person identified by face recognition as "Bill Miller" 2. By accident, I assign the keyword "B" instead "Bill Miller" 3. The keyword "B" (which is the root note for all names starting with "B") does not have any other photos assigned 4. I now realized my mistake and change the "B" assignment into "Bill Miller" 5. ERROR: Lightroom now realizes, the "B" has not have anymore photos assigned the deletes "B" INCLUDING all keyword underneath B 6. As a result, all my keyword underneath "B" (which are in fact all people starting with "B") are deleted!
That is a very serious bug since it can (and did for my) destroy a lot of work and you do not realized that the keywords underneath "B" disappeared.