Heya Toliakin,
If this persists past a UI reset, it probably means there’s a left behind lua or console variable messing with your chat. Are you running any addon managers? (Twitch/wowmatrix/tukui client/etcetc) If so, let’s uninstall those and try resetting the UI again. Once that’s done, type this into your chat box, press enter, and see if it fixes.
/console cvar_default
If it does, make sure to log out safely so it accepts the changes. You may get some notifications that you don’t have permission to perform the function, but the important bits should be run nonetheless.