Official Oracle APEX 23.1 documentation page (Creating Help for Your Application) says:
"“To display Help text for the calling page, create a dedicated Help page with a Help Text region. Then link to that page and include the calling page number in the Request value.”
Done, and done, like many times before. However, what the document does not say clearly is that “Request” is not passed into session state by default in modal dialogs.
In other words, one can not use a modal page (or drawer page) to display HELP_TEXT from the calling page.
If everyone knew this already, great. Trying to help others so you don't waste several hours (like I did) on trying to figure out why it's not working per Oracle instructions.
Vojin