Hard to identify something online without actually handling and examining the gun in question ...
However, it doesn't seem to be uncommon to have folks experience this sort of thing. If the hammer is cocked in single action when you're trying to remove the slide (and many .45 pistols will cock the hammer when you retract the slide for field-stripping), then the hammer will have to be decocked in order to easily remove the slide. This can be done by pulling the slide forward, but that requires the slide press down on the sear release lever and displace the sear nose from under the hammer's single action notches (there are 2 of them, separated by a space). This is ordinarily intended to be done by rotating the manual safety body to push down on the top of the sear release lever, smoothly and by design, but the rear of the slide can do the same thing, although not always as smoothly or as easily as when it's done by the intended part.
Another way is to carefully hold the slide in the 'normal' position, as if still pinned to the frame (meaning the slide stop lever has already been removed), and then decocking the hammer using the decocking lever (down & up, returning the lever to the 'ready-to-fire/off-safe position'), lowering the hammer so the slide can then be pulled forward off the frame.
Sometimes a gun may have an accumulation of fouling inside the frame's fire control parts, usually caused by neglect and improper cleaning practices (sometimes resulting in a congealed goo & sludge which can create interference with the intended movement of the parts).
Sometimes a sear release lever may have become burred or damaged in some manner, too.
Armorers are trained to check the tolerances of the sear release lever periodically to make sure that it remains within normal spec, and replace it if it wears out of normal spec. It's a fitted part, BTW. You can occasionally see among firearms forums where someone unfamiliar with working on S&W pistols may discuss how they have WRONGLY tried to 'smooth' the top of the sear release lever, thinking to make it 'easier to remove the slide' but not realize that this can ruin the tolerance of the fitted part of the lever (at the bottom, where it contacts the sear nose edge) and result in difficult or late decocking, and even take the part out of spec so that the hammer can no longer even be decocked in the intended manner.
Sometimes it's the accumulation of poor cleaning practices, too.
I had someone bring me a relatively new pistol which suddenly wasn't consistently allowing the trigger to reset in SA mode during firing. Not a good thing, obviously. When I disassembled the gun I found I could almost pour excessive lubricant and goo out of the frame, and the parts were literally sticking together inside the frame because of the incredible amount of nasty sludge which had formed. No wonder the gun wasn't running normally. I discussed normal cleaning practices with the fellow to whom the gun was issued. Once the gun was cleaned, reassembled and properly lubricated, it ran just fine.
Sorry I can't know what's happening with your gun. If we lived near each other I'd offer to examine it as a courtesy.
I'm not an 'expert', though. Not a gunsmith, nor a factory technician. Just a LE armorer.