I believe that is working correctly. A deform (FFD) can only be applied to 1) the mesh the deform was keyed for, or 2) any linked meshes for that mesh that have "inherit timelines" checked.

As always, we recommend not using deform keys as much as possible:
https://esotericsoftware.com/spine-keys#Deform-keys
Use bone weights to deform meshes, it's better!