You know those numbers and markers on the paper backing of 120 film. Well i can see them in my exposures after I develop my film.

I'm using Ultrafine 120 film, yes I know it's cheap. The paper backing is back and the numbers and markers is white. But how does the image of the markers and numbers appear on my negative?

It's more noticeable with long exposures (anything over 4 seconds is obvious).

Is it the light burning through the red window on the back?

It's just so odd that somehow the numbers can be seen through the black paper backing onto the film negative