Apple Watch 9 only charging to 80%

I have a two week old Apple Watch series 9. Went on vacation and I’m using a charger with a USB connection and a small cube charger, 5 watt., not the USB-C charger that came with the watch.

My watch will not charge beyond 80% and I’ve turned off battery optimization. It just stops at 80% even after being plugged in for 10 hours and 3 different cycles. It also gets pretty warm. I’ve tried 2 different 5 watt cube chargers and same issue.

Is this caused by the fact that I’m not using the 20watt charger that came with it?