Seems like the issue has been fixed in Bluez 5.55.
From the changelog:
Fix issue with handling AVRCP notification events.
Fix issue with handling AVRCP list player attributes.
Fix issue with handling AVRCP category 1 player settings.
Fix issue with handling AVRCP media player passthrough bitmask.
I'm on Ubuntu 18.04 and Bluez is at 5.48.
I had created a server with only an IPv6 network interface.
184.108.40.206 is an IPv4 address, so I couldn't connect to it from the IPv6 network interface. Same for the Docker server.
In order to get IPv4 access to/from my server, I will need to attach an IPv4 interface to it.