This seems to be because of recent changes made for [Link Removed]. Now that UNetDriver::bInTick is being properly tracked for derived classes, AOnlineBeacon::CleanupNetDriver is now deferring destruction of the NetDriver rather than immediately calling DestroyNamedNetDriver. When AOnlineBeaconClient::DestroyNetworkActorHandled is then called when attempting to destroy the beacon, the connection is still considered open, and so destruction of the connection/beacon is also deferred by setting bPendingDestroy on the BeaconConnection. However, the BeaconConnection isn't destroyed, since the NetDriver is shutdown at the end of the tick without ticking the connection again.
While destroying a NetDriver from a RPC is not considered a supported operation, this did work previously for beacons. If this is something that shouldn't be supported, it should likely be made more clear what is going wrong, such as with log errors or warnings.