But the paper boarding pass would still have beeped at the gate if you had been moved?
You are correct, that would have happened if I had scanned my original cardboard BP rather than my FF card.
I boarded the aircraft by showing the FA my BP and sat in the seat shown on it, not the seat I had been moved to (as I was unaware I had been "moved"). This caused an issue when the PAX who had been allocated my original seat boarded, saw me and aggressively demanded I move from "their" seat. They became more aggressive when I showed them my boarding pass with that same seat number. It took the FA and staff at the gate to sort it out.
Correct and also would've printed a paper slip with the new seat too.
As already indicated, I did not look at the slip; I just stuck it in my pocket (Can't read it without reading glasses which I don't normally wear walking around).
The situation was needlessly caused by staff at ADL.
Because of this incident I have not used the barcode on my FF card to board a flight since - that was the first time and the last time.
(Nor do I use my 'phone as it's easier to show the FA a boarding pass held in two fingers while juggling my carryon as I board.)