Different explain plan when using synonym?

I have a synonym pointing to a table and when directly accessing the table (ap_checks_all) queries return data as expected.  Usng the synonym (ap_checks) no rows are returned.  Looking at the explain plan they both access the ap_check_all table but the synonym has a a 'null is not null' filter predicate.  There are a couple of possible hits which suggest aome alter session workarounds but these do not appear to work.  Has anyone come across this?