If the animation in a playable controls a particular property and you play a different playable that doesn’t control that property, it will act as if the other animation was animating that property to set it to 0 every frame.
This is why the “Write Defaults” toggle exists in Animator Controllers and something similar needs to exist in the Playables API for the exact same reason.
I’ve also tried playing around with Animation Jobs, but there doesn’t even seem to be a way to detect whether or not a particular playable is animating a property (other than the hacky assumption that value == 0 means it’s not being animated which will obviously not always be a viable workaround).