Sometimes the second mast section starts to raise before the first one has reached it's limit. The top sections can also stay extended too long as you are lowering the forks. As the lower section starts to lower, the top sections will free fall slightly to catch up.

I am thinking it's a valve sticking, causing the pressure not to divert properly. Does someone have more info? If it is a valve, where is it located (in the union connection?)

If you extend the mast fully before you start to use it, it seems to work ok. It has plenty of oil, looks pretty clean. Haven't tried changing the filter, since the pressure seems fine.