UPDATE
I found out what the difference is between those two links and why one of them isnt working when the anchors are active on the same page as the links are.
Working anchor link: /methodieken/#zkm
But in reality the link is: /over-ons/methodieken/#zkm
So it works when not correctly linked.
With the second links I had the real link as anchor: /producten/#ppr but that just don’t work on the page that has the anchors. So I made up my own non-existing link /p/producten/#ppr and voila the anchor works.
For now it’s great that I got it working because the site will be live soon, but ofcourse I would like to have a more permanent solution to this issue. Making up fake links to get something to work is not my kind of workflow ๐