I just booked an ADR and then went to book a FP at the same exact time and all it did was include a note at the bottom of the FP screen that I had an ADR at the same time. Nothing popped up, no alerts at all.
These are both on the website/browser.
Tried on the app and it doesn't do anything, no alert, no box to even notify you that you have something overlapping at all.
So yeah, whatever you are seeing is a glitch and unfortunately, your glitch, not one that others are having. That's why I was asking if you could get a screen shot or snip of the image or message. Maybe someone else would have seen it or otherwise recognizes it. Because something unusual about it for sure.