Dear Christian Rupp,
Thank you for reaching out to the Microsoft Q&A.
This is a fascinating and frustrating problem, and your detailed description is excellent. You've correctly identified the key variables: the ARM64 architecture, the custom keyboard layout, and the specific type of text fields where it fails.
The root cause of this issue is almost certainly a compatibility problem between the very old Microsoft Keyboard Layout Creator (MSKLC), modern Windows applications (UWP/XAML-based), and the Windows on ARM architecture.
The most reliable fix is to stop using MSKLC and switch to a modern tool like EPKL (eDEX-UI's Portable Keyboard Layout) or KbdEdit (a paid, commercial alternative to MSKLC) to rebuild your layout. This addresses the root architectural incompatibility.
I hope this could help. If you have any further issues, please let me know.
To assist others who might have similar questions and to help us improve our support system, we kindly encourage you to "Accept the answer" if it successfully addressed your concern. Accepting an answer lets other users know that this solution worked for you, and it also helps us track the effectiveness of our support efforts.
Best regards,
Bryan Vu | Microsoft Q&A Support Specialist