How does RODC connection failover work?

If the bridgehead replication partner of an RODC becomes unavailable, the KCC on the RODC builds a connection to another partner. By default, this happens after about two hours, which is the same for a writable domain controller. However, the FRS connection object on an RODC must use the same target as the connection object that the KCC generates on the RODC for Active Directory replication. To achieve this, the fromServer value on the two connections is synchronized.

However, the trigger for changing the fromServer value on the FRS connection object is not the creation of the new connection; instead, it is the removal of the old connection. The removal step happens some hours after the new connection object is created. Consequently, the fromServer value continues to reference the original partner until the old connection is removed by the KCC.
A side effect of this is that while Active Directory replication works successfully against the new partner, FRS replication fails during this period. The additional delay is by design—it avoids causing FRS to perform an expensive VVJoin operation against the new partner, which is unnecessary if the outage of the original partner is only temporary.