Here is a bug (https://gitlab.freedesktop.org/pulseaudio/pulseaudio/issues/525) initially reported for PulseAudio stating that sinks for headsets are not always connected. Especially Sony headsets are mentioned throughout the bug. It turns out the reason is that the a2dp profile is not properly advertised during connect. Lots of people have tried different workarounds with varying degrees of success.

According to the Bug Report finally a fix was applied to bluez to connect available sinks even if not advertised (https://git.kernel.org/pub/scm/bluetooth/bluez.git/commit/?id=477ecca127c529611adbc53f08039cefaf86305d). The fix will be part of bluez>5.50. However the last release of bluez was on 3rd June 2018 (http://www.bluez.org/category/release/), so it could still take a while before users have access to it.

It tried the fix together with a Sony WH-1000XM3 headset and it worked exactly as expected. The a2dp sink appears on connect and PulseAudio immediately switches the streams to the headphones.

So I suggest applying the fix to the Arch Linux bluez 5.50 package. It is really frustrating if new headphones that are simply expected to be plug-and-play require rather technical workarounds which most users are not capable of.

Regards, Peter.