Apple Watch keeps locking itself and does not unlock on wrist raise

Your report suggests two problems, uncommanded locking (wrist detect) and failure to detect a wrist raise. These functions use two different sensors and I think it is unlikely both would fail at the same time. Some shared function in software is a possible suspect, or possibly a failed wrist detect sensor is blocking wake on wrist raise. That suggests two possible solutions,

• Clean the watch to be sure the wrist detect sensor can see you. Apple's official advice is in How to clean your Apple Watch - Apple Support but basically a good wipe around the back of the watch with the legendary microfiber cloth should be good enough.
• Deal with any software glitches. Forced reset the watch by pressing and holding both the Digital Crown and the Side Button (together, at least 10 seconds) until the Apple logo appears, then release the buttons and let the watch reboot. If that doesn't work you could try unpairing then re-pair and setup again.

The method of last resort is to have Apple check your watch for a problem sensor. Contact Support to arrange this.

I would suggest you reset your watch. Press the crown and side button in untill the Apple logo appears. Also Check your Watch is updated to 5.2.1and your iphone is running ios 12.3.1. Finally still having issues Unpair your Watch and repair it.