I think the key difference is I don’t plugin my custom tool tip in the designer menu, but I create a binding and then in that function create my widget. That’s probably why it works for me.
Pics to clarify:
https://i.imgur.com/eOuwHUZ.png
https://i.imgur.com/iEW4GIn.png
In this case, the IMG head Slot BG is whatever object you are hovering over/made the binding on, and its tooltip text is whatever you input on the designer menu under tooltip text.