Here's my experience based on a handful of cases, none of them in the last two years.

I used the RJ and previous QF sites to switch to WT+ bassinet seats before they were available. I did so knowing that I could potentially get shifted but I never was. I also did it relatively close to departure (days not weeks out) and when there were multiple bassinet seats blocked (not occupied). I figured most families with need of a bassinet would book more than a couple of days out, and even if there were a short notice booking it could still be accommodated without needing 'my' seat.

I have no way of knowing if my actions deprived an infant of a bassinet spot but I think it unlikely. I was never moved.