I retorqued the headnuts and placed one extra ring under the one nut that I found 'loose' again. Idea was that maybe the tread on the one with the most elasticity was not perfect and the extra nut ensures the use of cleaner thread. Or if the nut bottomed out, the extra ring gives more space and makes sure the nut clamps down on the head for sure.
In any case, the elasticity is now gone...
I have not taken the head off. Just retorqued with the head in place.