BLE whitelist

Any idea what could be involved and how to fix it?

I'm afraid Whitelist isn't of much use with recent Android phones. If you click 'read more' next to https://banglejs.com/apps/#setting - it says:

NOTE: iOS devices and newer Android devices often implement Address Randomisation and change their Bluetooth address every so often. If you device's address changes, you will be unable to connect until you update the whitelist again.

The Address Randomisation is a form of anti-tracking stuff phones started adding, but the downside is obviously if they keep changing their address there is no way to identify the phone :(

The best bet right now is to require a passkey, but failing that if you turn off 'programmable' then the worst anyone could do is to pop a notification on your watch screen - they couldn't actually change any config on your watch.

Also worth noting that usually your phone will stay connected to the watch, and that itself stops anyone else from being able to see or connect to the watch.