could it be that you have more than one keyboard input language installed and
by accident this is changed during notepad++ editing?

Eko

My best guess is either you’ve got some other app running that’s highjacking keystrokes (intentionally or otherwise), or that a plugin is highjacking keystrokes.

If you reboot and launch windows in safe mode, with Notepad++ run from command line or Windows Run dialog with the c:\program files\Notepad++\notepad++.exe -noPlugin option (using correct path notepad++ executable), does it still have the problem?

It might be helpful if you provided us the output of the ? > Debug Info menu (you can click on the copy-to-clipboard link, and paste it in a reply: that will give us your exact version of Notepad++ and environment, along with the list of active plugins.

I can’t see this accidentally happening, but if somehow Shift+[ got tied (in Shortcut Mapper) to a function that often seems to do nothing when you invoke it??