No $SYS and # subscription without using localhost

[spiech1]

Hello!
I am facing a strange behaviour.
I would normally use the default subscriptions to automatically subscribe to $SYS/# and # using the hostname in the 'host' section.
This worked perfectly for a while. Now it does not work anymore. I can only selectively subscribe to other(!) defined topics.

Thing is that if I ONLY change the host field from the hostname (or IP) to 'localhost' it will automatically discover $SYS/# and #.
No other changes necessary, just the host field. It will still connect, but not receive messages for those topics.

Do you have any idea what would prevent MQTT Explorer from seeing $SYS and # when using the hostname?
Thanks in advance!!