I don't have a great explanation for this. If the disc splines were seized, it wouldn't come off the flywheel at all.
There is a certain amount of flexibility in the disc center hub. I don't believe the splines are "seized" but might be too tight on the splines. With the pedal depressed, you should be able to pry the disc away from the flywheel surface.
I would pull the trans and apply some spline lubricant to the splines before reinstalling.
