This is correct. There’s a total black border on all 4 sides. Because the black bars are rendered into the top and bottom of the video. On a 1080p display it full screens fine but with a display with the native wide screen aspect of the actual video I can’t actually have it fill the screen.

But only on paramount+

This used to be a problem with VHS/DVDs when widescreen TVs first came out. TVs often (and maybe still do) had a mode to zoom in to cut them off.