When you select text, the F6, F7, F8, F9, and F10 keys are reserved for language functions if you’re using the Microsoft Japanese Input Method Editor (MS-IME) for Windows 10, Version 2004 or later.
The Japanese IME allows you to enter Japanese characters and symbols on a physical keyboard. If you’re in Japanese mode with one of these Windows versions, the F6 – F10 keys can be used in apps like Notepad to change character types.
To use the function keys for other shortcuts in apps (for example to start processing in C#), make sure you don't have any text selected while the IME is in any of the Japanese modes. Or you can revert back to a previous version of the Japanese IME.
This update creates a consistent experience for apps that use the legacy IMM32 API.