I don't know if this is another possibility, but if the last images were loaded into the reel first (as in directly out of the cassette), and were not wound onto the reel well separated from each other, uneven fixing, leaving yellowish (can't tell the color from the contact sheet) emulsion behind in the unexposed areas. These areas would block light during print exposure and could look like this on a contact sheet. Does the film look yellow in these areas, or exposed?
Another possibility is that the cassette was dropped, or compromised after removal from the camera, fogging the last few frames.
I've never opened the back with film in the camera, but it would seem that if you did that, toward the end of the film, there would be an area 3-4 frames long that would be solid black on the film (white on the contact) that would be the part of the film that was stretched between the cassett and the take up spool at that moment. I don't see that in the contact sheet.