External ultrawide monitor with black bars when playing videos

Hello,

I have an LG 29WK600 connected my Macbook Pro 14" 2021 thru HDMI, using Sonoma 14.1 OS. The monitor displays image correctly all times, getting 100% of the screen. But when I put a ultrawide video to play in full screen mode (which I would expect to fill the whole monitor), it displays black bars on all sides. This happens with all softwares I tested (ie: Apple TV+ app, Opera or Firefox browser displaying YouTube videos)

Do anyone knows how to fix that?

Thanks!