I'm using an iPad Pro and my facial recognition also suddenly stopped working at version 15.3. I spoke to several helpful support staff who had me try a reset to factory settings but do far nothing has helped. It seems to be a software issue that has not yet been solved in the 15.4 version (although Apple claims Face ID gets more accurate result mentioned here). Check your phone while taking photos in portrait mode. Mine stopped doing that as the same time - apparently numerous users have reported a similar problem, so they may be connected.
I’ve heard from support that this issue is being flagged for the engineering department/ software gurus to take a look at.