Hello Jelle,
Normally, i would expect when confronted with this kind of weird behaviour a note in known issues in the release notes, but as you can see there is not:
https://docs.mendix.com/releasenotes/studio-pro/9.18/
So i dont think there is currently a real explanation for it, but if you really want to know you can always file a ticket for these kind of behaviors.
Good luck!