We're having the same issue. Code that worked on Windows 10 is now not working in Windows 11. In addition, I see that MS sample code for implementing custom dictionaries in GitHub isn't working: https://github.com/microsoft/WPF-Samples/archive/refs/heads/main.zip with the project in Documents/Spell Checking/CustomDictionaries.

In the past in Windows 10 we saw problems where new words in a custom dictionary weren't being recognized due to temporary .dic files building up in the %TEMP%\wpf folder. But clearing these files is no longer fixing the problem. We've also tried clearing Registry entries in Computer\HKEY_CURRENT_USER\Software\Microsoft\Spelling\Dictionaries to no avail.