The user guide mentions Flag.FORCE_WRITE_LOCK in the context of pessimistic transactions but it does not say what this does in non-transactional caches.

In non-tx cache it does not work since when on backup owner, the get() just reads the value. It does not go to primary to lock there. When on non-owner, it reads from some other owner but not necessarily from the primary. The flag does not force the command to acquire locks remotely.

To put it in other words, it works only scarcely = it does not work.