I had the exact same question and after a lot of troubleshooting(I am also on the same setup (UBC C Charges the Mac and gives out display to the monitor) :

If you want to play it on Google Chrome effectivly :

1. open chrome>3 dots on the left >Settings > System (Left hand side, bottom), Use graphics acceleration when available ( Turn ON) and it will work fine!

If you want to play it on safari seamlessly

• they have update safari, its already fixed.

Entire Mac is not in power saving mode( I know you have mentioned already) Last attempt if nothing works out : Reduce the monitor frame rate to 60 Hz

I am also on the same setup (UBC-C Charges the Mac and gives out display to the monitor).

If you want to play it on Google Chrome effectively:

1. Open Chrome, select the three dots on the left, then select Settings > System (left hand side, bottom).
2. Turn on "Use graphics acceleration when available".

After these changes the video should work fine.

If you want to play it on Safari, Apple has updated Safari and it's already fixed.

Make sure your Mac is not in power saving mode  (I know you have mentioned already).

Last attempt if nothing else helps: Reduce the monitor frame rate to 60 Hz.