My watch8 keeps locking

The Unpair/Pair routine should have cleared any software or settings issues. That seems to leave two options.

• Check you are wearing the watch with the band (watch strap) fitting it correctly to your wrist. See the description in Wearing your Apple Watch - Apple Support
• A possible hardware defect. I will simply remind you that Apple's standard return/refund policy is 14 days. This would allow you to reject a defective device and purchase again (in theory) but it is more likely a call to Apple Support and a repeat visit to the Apple Store will get a problem device swapped out. If you already started a support case they should be able to go back to the notes from your first visit and see the attempted fix did not work.

Either

• your watch strap is loose
• you have tattoos
• you watch is faulty

Position on the wrist should not affect wrist detection which works even a couple mm away from the skin.

With a loose strap, slowly very slightly pull the watch away from your skin and see at which point the blue lock symbol lights up.

Describe what you mean by Lock.

Same for me. This issue is discussed here too : Apple Watch Ultra locking while on wrist - Apple Community