Works in 5.5. Are you sure you are using the correct blueprint node to change the skeletal mesh’s asset?
It should look like this:
Yet in yours it looks this way:
Notice the exclamation mark with a cog on the right hand side of the node. It might be the wrong version of the blueprint node (maybe you are setting the variable instead of the asset of the variable)
Edit:
Narrowed it down:
You must have had context sensitive turned off and chosen the wrong node:
You chose a component node instead of the node that is called on the component itself.
Try turning on context sensitive blueprints and drag off of the Skeletal mesh appearance to get the correct node.