Apple Watch stops charging randomly

I have an Apple Watch series 9. About 5 days ago, I took it off the charger from overnight. The lightening bolt showed up. It was still on 20% so it eventually stopped charging. After another night, it stopped charging at 12%.

Now it will only charge if I restart it. But then it stops charging again before reaching 100%.

The watch is fully intact, the charger charges my series 6 just fine. I’ve cleaned the watch several times. How do I fix this?

[Re-Titled by Moderator]