Apple Watch Series 10 won't charge past 80%

You can disable this feature and charge your watch to 100% each time you put it on the charger.

Have a look at the related article, in which it is also being explained, why Apple introduced this option:

About Optimized Battery Charging on your Apple Watch - Apple Support

Sounds like you have optimized charging enabled. When you do that, it learns your habits and only charges to where it needs to be for you to get through your day to avoid unnecessary wear on the battery.

From what I understand you only want to charge it to 80%. Every so often it will to 100% to recalibrate the charging/battery. Best bet is to charge to 80% and that will extent the over all life of the battery.