As explained in my post: I have already turned off all Windows autocorrect features in Settings. Same for registry (everything is zero, here). So, not yet solved.
Some apps like Asana, Todoist or others highlight English text with red lines. It concerns apps that can also be used via internet browser. Context menu reveals that this is a spell check (name of the spell checker app not provided). I have turned off ALL spell checking features of Windows 11...