In version 3.7.0, VRChat Constraint sources were implemented in a way that reset their values to the defaults (no assigned transform and a weight of one) as soon as they were loaded if no defaults had been applied to them yet. Version 3.7.3 updated them so they no longer behave in this way to better support user made SDK tooling. Downgrading a project from 3.7.3 or later to 3.7.2 or earlier triggers applying the defaults in the old way, which overwrites any pre-existing values.

The behavior you've noticed is known and is not expected to change. We do not generally recommend downgrading the SDK version used by a project.