[SpirosZ1000]

I don't think that your problem is compression.A little higher comp. would be a problem while trying to start the engine cold with the starter and would cause engine problems when using bad fuel and for longer periods of use from what you are saying, not keeping you from putting the kicker down and brake them like that.
I would suggest to try some things before you end up dismantling pieces.
1st open the the radiator cap and see the level of your coolant because there is a chance ( seen it with 2018 250 model recently) coolant gets to your combustion chamber and causing hydraulic pressure (effect).
Solution open your cylinder head and inspect the o-rings and while you are there inspect the cylinder for cracks( rare occasion but could have happened for many reasons)
2nd take the spark plug off (observe it) and try turning the engine with your hand over the plug hole to see if anything liquid comes of other than gas (not much) Maybe for some reason your engine overflows with too much fuel.(not so likely)
3rd something is binding. Take the front pipe of and watch your piston for anything unusual. (scorches, broken piston rings or out of their place).

Report what you find and hope you find it soon.

p.s. In case you need cylinder head o-rings try from KTM same bore.
Gas Gas o-ring weren't the best at least the older ones but KTM's are thicker and work great with less problems.