Unity rotation constraints that have a single source with a weight smaller than 1e-6 set the objects rotation to world identity regardless of its parent orientation.

Parent constraints inherit this behavior too. For their position part it however acts the same as position constraints which multiply the world space position with the source weight when there is only one source.

This means a parent constraint with a single source of value 1e-35 will zero out its world space rotation since the weight is below the threshold and multiply the position with the tiny value effectively putting it at the world origin.

I made a test avatar to showcase this: avtr_00a3087e-fcd8-400c-bf38-f88e54663dac

Setup and testing in unity (works like this in vrc on live too): https://www.youtube.com/watch?v=wp0QDo-CHPo

VRChat open beta ingame showing wrong behavior: https://www.youtube.com/watch?v=d6Nc-WFsno8