Just wondering if anyone has an explanation as to what causes these things to stick so hard, that even a claw hammer sometimes won't budge them.

Does not look to be anything in either the bore or the lifter to catch !

 

I have heads with most stuck in them.

And before someone says carbon and gums and varnishes from the oil ...this thing has had oil changes between 2500 and 5000 km intervals done based on time rather than distance, and reportedly has less than 100,000km on them.

 

Overheat maybe ? The timing belt cases were suffering melt droop and have been chucked

I would say that the melted timing cover is your best clue. The heat was probably a factor.

I noticed a few that were stuck in one head - the wire spring clip seemed to have worn / deformed the edge of the sleeve a little, rolling it into the groove on the lifter.