Based on my SQ-A -- with the film back off, you should not be able to push in the back release button, unless you also push in the silvery interlock pin at the lower left of the camera body (viewed from the rear). If, with the back off, you can operate the back release without fiddling with the interlock, something is likely amiss in the body.
I suppose it could be gunk impeding motion somewhere. Either the body side of the interlock doesn't have enough spring tension to keep the interlock pin pushed outward, or the matching interlock on the film back is too stiff (or bent) to retract when the dark slide is out.
Edit: Just after I wrote:
I looked at a photo I have up of a film back and it doesn't have that extra little silver tab, so apparently not all SQ-xx backs have the dark slide non-removal interlock. A few production changes over the history, I guess.You also should not be able to remove the dark slide from the film back while it's off the camera either. With the film back off, the dark slide can be removed by pressing a little silver tab just up the dark slide side from the interlock pin. It is apparently depressed by contact with the body when the film back is mounted.
(Clever but complicated these buggers.)