Apple Watch locking while on wrist

I own an Apple Watch 4 (Model A1978) and recently it's started to lock itself while on my wrist. I've looked at previous community posts with this same issue, but none of their solutions help.

I don't have any tattoos on my wrist, I have cleaned the sensor, turned off Wrist Detection, turned off Sleep Mode, and restarted/re-paired the watch and my iPhone. It still locks itself every 10-15 minutes. I've had to turn off the passcode in the interim, but it isn't a solution and isn't a great workaround.

Are there any steps I'm forgetting or has anyone else had the same issue?