This is a known (and nasty) bug in the taxonomy hierarchy caching code:
http://core.trac.wordpress.org/ticket/14485
Basically, you have to force a refresh by deleting the option.
This is a known (and nasty) bug in the taxonomy hierarchy caching code:
http://core.trac.wordpress.org/ticket/14485
Basically, you have to force a refresh by deleting the option.