Jake, nice pics. Let's look closer.
So, it looks like the crank is jammed at about 20°-30° BTDC. The cam is about 10°-15° BTDC. At half rotational speed, that looks right. 20° at the crank equals one tooth of the cam sprocket. So, I'd say the cam timing is right.
View attachment 99507
Now we're entering the 'twilight zone'.
At 20° BTDC, the pistons still have about 2.5mm - 3mm of upward travel before they reach the top. With the camcover off, and the camchain loose enuff, the head/cylinder group can be nudged upward, possibly without being noticed.
Looking into the left cylinder's plug hole, you can see the piston top, and its direction arrow.
View attachment 99508
Looking into the right cylinder's plug hole, .....well, I'm not sure what's going on in there. Some sorta markings?
View attachment 99509
So, I'm thinking along the lines of some sort of foreign object in one of the cylinders, randomly bouncing around, then suddenly getting into just the right position to jam between a piston's crown and combustion chamber. Loosening the camchain allows the head/cyl to nudge upward slightly, giving the impression of being freed.
Maybe you could peek into the plug holes, see whut's in there, maybe pluck something out.
Better if the head is pulled, the piston tops examined, and the valves checked for seating/sealing and/or bent...