Because the primary does not have knowledge of which secondaries are currently connected via the spanning tree, it can not tell when secondaries are down, unless we depend on heartbeats.
We are currently increasing the heartbeat rate for secondaries with no sync source, but such heartbeats are ineffective for updating a primary's liveness table. The work for this ticket should be to make such incoming heartbeats update the liveness table just like outgoing heartbeats. We may need to include more information in the outgoing request so that the receiver has enough data.