I have seen where if you add a UDF to a panel and then delete the UDF without first removing it from the panel all the descriptions will drop off.

So you're saying if I had 5 UDFs with descriptions placed on a panel and I decided to delete one of them without first removing it from the panel that the other 4 UDF descriptions might disappear?

That may explain it, I haven't really made a habit of removing a UDF from a panel before deleting it (not that I delete many of them).

I noticed this situation last week and wondered myself.

I believe that your description of what causes it did apply in my situation.