Hi nimrodbartor,
Thank you for reaching Microsoft Community. Apologies for the inconvenience this has caused you.
Just to make sure that I get the exact picture of the concern: the black bars only appears when you are playing videos online, correct?
How about if you play a video that is locally saved on the device like if you take a video using the camera then save it locally on the device then play it, do the black bars also appears?
Also, is the image on the video being cut by the black bars or you still see the full picture of the video but its just that the screen is shrunk to lower size?
Please take a screen recording on the device or a video showing the screen where we could see what happens when you play a video then share the video here with us.
I'd love to provide you with the troubleshooting steps right now to fix the problem. I am afraid that we may have to wait for the above answer first so that we can provide the necessary information. Once we collect all the information, we will formulate the feasible resolution to help you with your case or if we need you to refer to the support that suits your problem.
Kind regards,
Marrion