Did you try to do the same retrieve in a microflow? What is the outcome there?
You could also try to do two retrieves in a microflow. One with the _IsKlantSpecifiek = false() and one with the Normering.Eigenaar_Klant = '[%CurrentObject%]' and then do a union over that list.
I know that you don’t want to use a datasource microflow. But it may help to find the cause of the issue.
See https://docs.mendix.com/releasenotes/studio-pro/8.18#fixes-1. There was a bug with CurrentObject in nested statements. Maybe also the case in your version?
Possibly remove the nesting in this case with acceptance of the performance drawback.