Bluetooth devices and VoiceOver speech

Hi everyone,
I hope I have posted this in the correct section of the forum - feel free to say so if not.
Yesterday I finally updated to iOS 13 on my iPhone 8 (so I have iOS 13.5).
I noticed that there are new settings which enable you to control the destination of VoiceOver speech when you have a Bluetooth device connected i.e. you can listen to music through a Bluetooth speaker while the VoiceOver speech continues to sound from the iPhone. Today I switched on my Bose Mini SoundLink Bluetooth speaker and I was very impressed with the fact that I could use that to listen to music but VoiceOver was still sounding from my phone.
However, I then switched on my Aftershokz Trekz Air Bluetooth headset and both VoiceOver and the music I was listening to came out of my headset (VoiceOver did not sound out of my iPhone speaker).
My questions are: does this feature only work with certain Bluetooth devices? Does it only work with Bluetooth speakers and not headsets? Does it depend on which version of Bluetooth the external device uses?
Furthermore, adjusting the settings is not as straightforward as I was hoping - if I wanted to adjust the setting to control the audio destination of VoiceOver, I have to restart both VoiceOver and my Bluetooth device to enable the changes to take effect. Are other people finding this is the case as well? Any tips on controlling the audio destination of VoiceOver would be much appreciated.
Thank you for all your help,