Have you tried switching the motion mode on Ash to ‘floating’?
The ‘grounded’ motion mode is intended for stuff like side-view platformers and the like, and I’m thinking that ash’s characterbody thinks Rufus is a moving platform and sticks to it once it’s on top.
(See ‘MotionMode’ in the docs)
Also, as for collision layers vs masks, the layer is which collision layers the body itself is in, while the mask is which layers it checks for collisions when it moves. So assuming that you want them both to be able to collide and not be able to move through eachother, then you’ll want them both to have layer 3 checked in their collision masks.
Here’s the section in the docs on collision layers and masks.