SUMMARY
Kate prefers to open files in tabs. However, it's also possible to launch a separate instances. Unfortunately, regardless of which Kate window was used last, new files opened from external programs (e.g. file manager) are always added to the initial Kate window.
Other applications (e.g. Firefox) open files in the last used/viewed window. That's more preferable behaviour for multi-tasking.

STEPS TO REPRODUCE
1. Launch Kate and open some file
2. Launch a new Kate instance (FIle - New Window) and open another file
3. Move second Kate instance to second virtual desktop
3. On the second virtual desktop launch file manager and there try to open text file with Kate

OBSERVED RESULT
Opened file is added on the initial Kate window so user have to switch back to another virtual desktop to see it


EXPECTED RESULT
Opened file is added to the last used Kate window

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Fedora 39
KDE Frameworks Version: 5.111.0
Qt Version: 5.15.11 (built against 5.15.10)