Pi Zero W - hci0 disappears and reappears

I’ve been running room assistant on 6 x Pi Zero W’s for a while now and there’s always been something that I’ve never got around to resolving. And that’s that every 30 or so seconds, the devices switch to not_home and back again. And because of this I never relied on it for presence detection.

At first I just thought it needed tuning with distances etc but today I got some time to sit down with one of them to debug it.

I found that running “hcitool dev” as quickly as I can shows that hci0 disappears and then reappears which coincides with when the device reports not_home.

Anyone else have this issue? I’ve read some people blaming the wifi and bluetooth using the same chipset and resorting to buying an external BT dongle. Is this what others have had to do and if so which dongle did you buy?