I am currently testing the night version J4 beta 6 dev on 12/13/2020. After importing an image into the media manager and changing the image to 16: 9 format. I notice that this image is defocused in square format in front.
I can confirm the issue
The problem is that the image has height and width set, the template define img as max-width: 100% but the height: auto is missing.
Can't reproduce here.