The iOS 15.7.1 Release Candidate seems to have disabled Face ID for certain users (via MacRumors). Users u/CockroachLate on Reddit and realviraj on Twitter and commenters on their posts have reported the iPhone 12 Pro, iPhone 12 Pro Max, iPhone 13 Pro and iPhone 13 Pro Max as affected models, so whether other iPhone models with Face ID are affected is unclear for now.
It seems like you can't fix it by resetting Face ID either, as that just continually asks the user to reposition the phone higher or lower, and then pops up with a "Face ID Is Not Available" warning that can't be worked around. That leaves inputting your PIN or password as the only way to unlock these affected iPhone models for the time being.
Users with any of the iPhone 12 or iPhone 13 models mentioned above are likely best updating their software to iOS 16, rather than sticking with iOS 15. If you're running an iPhone 7, iPhone SE (2016) or an older model, you won't be able to escape iOS 15.7.1 as Apple only offers iOS 16 for the iPhone 8 and newer.
The best you'll be able to do is wait for the official release as it will hopefully have this bug fixed, although since none of these models offer Face ID, hopefully whatever coding error won't have a negative effect. Definitely do not try and download the release candidate for yourself either, unless you have a spare iPhone lying around that you don't mind being the victim of potentially dodgy coding.
Ideally though, you'd upgrade your iPhone to iOS 16 if you can. The latest revision of the iPhone operating system comes with lots of useful new additions. You can now customize your lock screen in completely new ways, unsend emails or texts, lift subjects out of a photo and get better typing feedback thanks to a new haptic keyboard option.