User loses all local group memberships if LDAP sync is unable to find the user, but the user appears again in subsequent syncs

(from original issue in CONF-28621)

Steps to Reproduce

1. Add a connection to LDAP in Confluence Admin >> User Directories with the Read Only, with Local Groups option
2. Sync the directory and make sure that LDAP users are returned
3. Add 1 LDAP user to a local group (membership)
4. Change the User Object Filter in the directory's configuration in Confluence Admin >> User Directories to a dummy filter, such as the following:

   (&(objectclass=inetorgperson)(cn=dummynonexistentuser))
   

1. Sync the directory again (Notice that the LDAP users are missing)
2. Revert the User Object Filter to the previous working filter
3. Sync the directory again (notice that the LDAP users are back, but their local group memberships are gone)