There's your answer;
You can turn wrist detection off but at a price;
Apple says;
Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance. The ink, pattern and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.
Wrist detection off;
Turning off wrist detection affects these Apple Watch features:
When you use Apple Pay on your Apple Watch, you’ll be prompted to enter your passcode when you double-click the side button to authorize the payment.
Some Activity measurements are unavailable.
Heart rate tracking and notifications are turned off.
Apple Watch will no longer automatically lock and unlock.
Apple Watch SE and Apple Watch Series 4 and later won't automatically make an emergency call even after it has detected a hard impact fall.
Make sure that you wear your Apple Watch correctly. The wristband fit has to be snug. Not too tight but not too lose either. If the Watch is too lose on your wrist, and the case doesn’t touch the skin permanently, erroneous Wrist Detection readings might occur. If you move your hand with a loose fit, the Watch can stand more than one second away from your skin, which is enough to trigger Passcode locking.Long sleeve clothing can also accidentally trigger Passcode requests. If the fabric lands in-between the back of your Watch and your skin, light probing won’t work properly and the wearable will consider that it’s not on your wrist anymore. This problem is also caused by a too loose band fit. If you wear a jacket you can tighten up a notch.
