I’m using version 2.3.3 as well, and my site is experiencing the same problem.
The post shows the appropriate categories ticked, but the front-end lists no categories, as does the back-end category manager.
When I go to add the categories that disappeared, magically, the post count per topic reflects the actual number. Then I refresh, and *poof* they disappear.
I just noticed this. I don’t know if this has anything to do with the “del.icio.us” xml-rpc posting feature.
