Image libary: Run a partial scan does not work. It runs a full scan

I have following settings of my server
Update my library automatically
Your library will be updated automatically when changes to library folders are detected.
Run a partial scan when changes are detected
When changes to library folders are detected, only scan the folder that changed.

I notice that -when something is changed in my picture library- plex starts a full scan of my whole picture library. This is annoying because I have other applications that index the images and store a file locally. This triggers plex every time

Is this a bug?

Kind regards
Bart