Switching on connect is an intentional design choice for Ubuntu. It's been like that for a few years although PulseAudio 13.0 in Ubuntu 19.10 has introduced extra sensitivity with respect to HDMI events...
If you would like to disable it then please just comment out this line:
load-module module-switch-on-connect
in /etc/pulse/default.pa
So this isn't a bug, but it does seem to overlap with bug 1847570 so please see that too.
Switching on connect is an intentional design choice for Ubuntu. It's been like that for a few years although PulseAudio 13.0 in Ubuntu 19.10 has introduced extra sensitivity with respect to HDMI events...
If you would like to disable it then please just comment out this line:
load-module module- switch- on-connect
in /etc/pulse/ default. pa
So this isn't a bug, but it does seem to overlap with bug 1847570 so please see that too.