Hello Melvin,
That's really odd, and I don't think the functionality isn't there, but rather is not exposed, I really can't see why it wouldn't work.
Check if you can use the "Show validation message" on a member that isn't on screen to display a message (This should show a pop-up since the member is not on screen) - I know it's not perfect but perhaps you can use it for a while if it works.
Let me know if the above works, it's always behaved as such via microflows.
Hope this helps.