Airpods and Apple watch intermittently stop working with 10.15

I have a BRCM20702 plugged into the m.2 slot on an Z390M-ITX/ac motherboard. I use a Vega64 graphics card.

On Mojave I had the same issue. I have now updated to 10.15.4 and the problem remains. I am using iMacPro1,1

Unlocking with Apple Watch and switching to AirPods works for a while and then stops working.

AirPods stop showing up in the audio menu. When unlocking there is a delay like it is trying to negotiate with the watch, but then just prompts for password.

I have a MacbookPro nearby and it always works.

Hand-off and AirDrop work without interruption. iMessage always works. My other bluetooth earphones are unaffected.

Can anyone help me troubleshoot this issue?