It is impossible to close the KMail window second time.
My setup is that kmail starts minimized to tray. And when I close window, it normally is still working in tray.
However, after upgrade to kde-unstable (I am on Arch, kmail package is 24.01.80-3), it behaves wrong.

STEPS TO REPRODUCE
0. Configure KMail so it can be closed to tray.
1. Start Kmail (may be autostarted at login) 
2. Click the tray icon to show main window.
3. Close the window. The window closes and the tray icon still shown (as expected).
4. Click the tray icon again to show main window.
5. Try to close window now.

OBSERVED RESULT
There is no reaction on close button. You can press Ctrl Q or just kill the window with Ctrl + Meta + Esc, but then the application terminates (no more tray icon).

EXPECTED RESULT
The window should be closed normally, and the app should continue working in tray.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.90.0
KDE Frameworks Version: 5.246.0
Qt Version: 6.6.1
Kernel Version: 6.6.7-arch1-1 (64-bit)
Graphics Platform: Wayland

Additional info:
After killing the window, relaunching kmail shows the message that it failed to start. Second start however normally start the kmail. And then the bug of unclosable window can be reproduced.