Bad image in tile view

I wrote my own client for JVB.
When I connect them, everything is fine. But if another participant enters the conference, a black mask is superimposed on the image in the tile mode. In full screen mode, the image is normal.

Why can this be?

If you right click on it and inspect elements, the dom tree should display. As you move your cursor around the tree, there should be a corresponding highlight on the browser itself. Do you see anything that matches the black cover? Full screen mode and tile view use the same html elements and generally the same logic for the thumbnails (because they are the same elements that get repositioned when tile view is toggled).

This is a screenshot at the same time in full screen.

In tile mode, only part of the image is hidden.
Everything else works as in other video frames

Are there any suggestions about the reasons for this behavior?
I can provide any information - browser logs, jitsi dumps, etc

Yes, right click and inspect element and look at the elements tree to see what element the black is.