In particular, it fixes the screen time bug.
Apple released the third beta of iOS 13.3.1 yesterday, which fixes a few things and fixes the screen time bug in iOS while also preventing children from bypassing parental controls. On the functionality side, you should not expect anything new.
In parallel, Apple has deployed the third beta of iPadOS 13.3.1, tvOS 13.3.1 and watchOS 6.1.2, which do not seem to bring major new features either.
Remember, the apple now has a bigger grip on its OS, no longer allowing to revert to old versions of its software, not in any case more than one or two releases.