Created attachment 119830 [details]
sample project with the described class and interface names, that can be used to reproduce the issue

SUMMARY
the implementation / realization association can only be set on classes that are listed after the interface in the tree view (sorted by name)

STEPS TO REPRODUCE
1. extract attached java code
2. import *.java files from subfolders of src/main/java  
3. drag interface QuakVerhalten from tree view to class diagram view
4. drag classes Quaken, Quietschen, StummesQuaken from the tree view to the class diagram view
5. check association arrows. only the classes that come after the interface have the implementation arrow. the class Quaken does not have the realization arrow. When drawing the association manually and double-clicking to edit the association, the Realization association is not available in the combo box.  

OBSERVED RESULT
not all implementations of the interface QuakVerhalten are displayed with the correct realization association. The classes with a class name that is before the interface name can not have the realization association. Only after renaming the classes, the realization association can be used.

EXPECTED RESULT
All classes that implement an interface should show as realization of the interface regardless of their name.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
I also tried to reproduce this issue by manually creating an interface test and implementing classes atest and ztest, but I could not set the realization association at all. If this is a usage error, I am extremely sorry.