EXACT STEPS TO REPRODUCE
1. Run K3b.
2. New Project/New AudioCD Project.
2.  Add audio files, etc.
3. Save K3B project; it recommends AudioCD0, with no extension of .k3b

OBSERVED RESULT
Then, try to re-open project, browse but no projects found
Manually rename project from xxxxx  to xxxxx.k3b,
project can then be found and loaded. I only stumbled on this requirement.

New, Clean Fedora 38 installation; run into this same issue on multiple systems.

I cannot locate a "automatically add extension" setting, or no longer exists, as recommended in this very old bug report; 
https://bugs.kde.org/show_bug.cgi?id=138525

Should it NOT recommend AudioCD0, but AudioCD0.k3b?

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma:                 Up to date Fedora 38, Cinnamon Spin, KDE Plasma 5.110?
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION