The memcache was not invalidated by NSS responder
When you add or delete a user, in the background the command sss_cache is run to clear the sssd cache. This should normally take less than a second. But if sssd is busy or the system overall is overloaded, it can take much longer to clear the cache. Typically this happens if you are adding or deleting users in large batches. The message is otherwise harmless and you should find that the user was correctly deleted.