KMail doesn't sign messages by default

Hi,
in KMail settings the checkbox “Sign all messages” under “Settings->Configure Kmail…->Securitty->Encryption” is checked (double verified).
However when I write a new message or respond to a message the message is NOT signed by default. I manually have to click the “Sign” button.

Any ideas?

My system:
Operating System: openSUSE Tumbleweed 20240924
KDE Plasma Version: 6.1.5
KDE Frameworks Version: 6.6.0
Qt Version: 6.7.2
Kernel Version: 6.10.11-1-default (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 7 7840HS with Radeon 780M Graphics
Memory: 13,3 GiB of RAM
Graphics Processor: AMD Radeon 780M
Manufacturer: LENOVO
Product Name: 83AM
System Version: IdeaPad Pro 5 14APH8

Thanks,
Johannes