When you have 4 documents open and arrange them via Window > Arrange > Tile you get something like this:
Bildschirmfoto20201106um22.05.-a4e571eb-2645-4daf-95ee-ab04012be4fc-1788174832.jpg
(I used the application frame in this example.)
When you now hit Command+0 on one of the documents it zooms to 313 % which is the same as it would do when you only have one document visible on your screen. Which means that the document is displayed a lot too big for the currently given window. I mean yes the command is titled "Fit on Screen" and actually it's still fits on the screen, but nonetheless that's an awkward behavior. I would expect Photoshop to be smart enough, so that it zooms to about 151 % so that the document fit's to the window (like in the manually zoomed example > lower right-hand corner:
Bildschirmfoto20201106um22.06.-d3cf1b2b-2749-4d58-bd28-837ee54118f7-852247519.jpg
Maybe I miss something here, but I my opinion that's more a bug than a feature ...