I have added another hotkey action in Stream Deck that has been set to press F4 (the shortcut I set in NotePlan’s preferences for system-wide search) and that works fine.
I was going suggest downloading ShortcutDetective from Irradiated Software - Labs to make sure nothing else is using that keyboard shortcut, but that doesn’t make any sense if it works without the Stream Deck.
Try assigning another keyboard shortcut and see if that works any better?
I’m not familiar enough with NotePlan to know if today’s note is a standalone note that can open without opening the main app, but if the intent is to press the button to switch to NotePlan, and then open Today’s note, you could use assign a multistep action to a button on the Stream Deck (first action switches to NotePlan; second action opens Today’s note).