If you're at all mechanically inclined, you can have a look inside the lens to see if the washer at the rear of the optical module is actually there.
All you have to do is unscrew that collar at the back of the lens with a spanner wrench (do not scratch the rear element, please.)
The optical module will come out easily (nothing will fly away, don't worry.) Before you dismantle the lens, cap it and place it, front element down, on a table. Lift the focusing helicoid/barrel off of the optical module and and look for a brass ring around the barrel that holds the rear element. It should be like a washer, a millimeter or so thick and perhaps 1.5 inches in diameter.
If that is missing, that is the cause of your backfocus. You'll have to find one in order to properly focus the lens, there's no way around it. This is the only way the lens can be adjusted and the brass ring has to be a certain inner as well as outer diameter or it can interfere with the barrel follower that guides and limits the focusing helicoid. Too thick and it can adversely bind the lens, too thin and it can get bound up between the optical module and the part of the focusing helicoid where it is seated. The ring also needs to be present in order for the aperture tab and the tab that guides the optical module not to get bound up or bent.
Good luck!
Phil Forrest