This seems to be a repeat of bug MC-132898.

This is on a multiplayer server.   When I stack two gravel blocks and raise them with two pistons (as in the gatehouse to raise a portcullis build) the top block breaks when they fall.   This does not affect the gravel blocks on the existing gates I have.  The existing gates work normally, however, if I break a gravel block on the original and then replace it, it breaks from that point forward.