Lock can be opened with credit card even when deadlatch is fully depressed
If the strike bolt can be depressed by direct pressure when the small strike is not fully extended, then the lock is poorly made or broken. Replace it. 

You should consider using a lock of higher quality. If this lock has failed, then perhaps all other instances of the same make and model are on the verge of failing. In any case, test the lock before installation. 

If you really want to know the mechanical failure mode, you'll have to take the broken lock apart, and guess how it should have worked.