Another update: I let my speaker rest a bit with Bluetooth disconnected and after reconnecting to it and playing another track it once again muted after 180 seconds (this is with the speaker firewalled with no internet access). I did a few more tests prior and it seems so far that this issue is specific to when the following conditions are met:
(A) The speaker is connected to a wireless network with a valid IP address, gateway, and DNS servers, and;
(B) The speaker is thus able to communicate with other devices on the same network (thus features like AirPlay work fine), and;
(C) The speaker is unable to connect to B&O's services for whatever reason.
In my current test config I moved my speaker back to my main network with internet access to facilitate easier testing, but the speaker has been firewalled (nftables "ip saddr x.x.x.x drop") on my gateway.
On this configuration music playback mutes after precisely 180 seconds of the first track after connecting to it via Bluetooth.
If I remove the nftables drop -- thus allowing the speaker full access to the internet -- the issue seems to disappear. We can thus conclude that the speaker is throwing a fit and stopping music playback after 180 seconds if it cannot phone home to do whatever it is programmed to do.
c-larsen: I'd appreciate it if you could try and get a Balance set up in a similar environment to attempt to reproduce the issue. In my exact situation the speaker was firewalled right from the beginning (i.e. no internet access during setup) and I had to wait some time for the initial software update check to time out during the first time setup, but it did eventually let me complete the first time setup of the speaker.
Thank you for participating in this thread as well, much appreciated! Cheers!