I'd sure consider it could be hot ammo.Look at the case,like looking at the primer.You can even look at the primer.You can see signs of pressure there.Any shiny spots on the brass,like the brass extruded into the ejector on the bolt face? Same old ammo or did anything change with the ammo? Different bullet,new powder? Primer? Was the ammo sort of hot?(OOPS! You answered my question. I doubt factory loadswould create bolt sticking pressure,especially if both WW and Rem do it)
One other thing,kind of crazy,but could be.New trigger. A Rem has a trigger safety. When the safety is "On" it locks the bolt. Generally,a stamped piece of steel in the rigger cams up and engages a slot in the bolt.Sometimes the gunsmith has to make minor adjustments to get things just right.
Also,might be that 300 kicks hard enough somehow the safety is getting moved.
I'm not sure if this next one has any mechanical possibility,but on an older stock,sometimes wood compresses. A few things can happen when the guard screws are tightened.That can make the guard screws act like they got longer.A screw tip can run interference somewhere.And,the crushed wood plus tight screws can flex the action.
But,IMO,neither of those add up to what you are getting.Why only after firing?