The heartbeat monitoring is not working with Trollmoves Mirror

[pnuu]

Describe the bug
If a Mirror is used, the heartbeat monitoring doesn't work in either direction, to the Server and from the Client. This causes the Listeners to keep restarting.

To Reproduce

Run the commands shown here by adding heartbeat_alarm_scale = 1 to mirror.ini and client.ini configuration files.

Expected behavior
The Listeners should be restarted only if the connection is (temporarily) lost.

Actual results
This happens with every heartbeat cycle:

[2021-12-03 10:01:56,323 WARNING  trollmoves.client] Missing a heartbeat, restarting the subscriber to dict_values(['tcp://localhost:9012']).
[2021-12-03 10:01:57,324 DEBUG    trollmoves.client] Exiting listener tcp://localhost:9012
[2021-12-03 10:01:57,324 DEBUG    trollmoves.client] Starting listener tcp://localhost:9012
[2021-12-03 10:01:57,324 INFO     trollmoves.client] Subscribing to tcp://localhost:9012 with topics ['/foo', '/heartbeat/move_it_server', '/heartbeat/move_it_client']
[2021-12-03 10:01:57,324 INFO     posttroll.subscriber] Subscriber adding address tcp://localhost:9012 with topics ['pytroll://foo', 'pytroll://heartbeat/move_it_server', 'pytroll://heartbeat/move_it_client']
[2021-12-03 10:01:57,324 DEBUG    trollmoves.client] Subscriber <posttroll.subscriber.Subscriber object at 0x7f1c2ba86a00>
[2021-12-03 10:02:25,394 DEBUG    trollmoves.heartbeat_monitor] Missing heartbeat alarm!

Environment Info:

• Trollmoves Version: 0.9.0

Additional context
The monitoring works between Server and Client, but not with Mirror in either direction. So could be something really simple.