That’s not the way trimming works in Rhino. In order to get the trim command to work after the fillets are made you have to remove the back surface. So why add that back surface in the first place? It just makes it designed to fail.
People who use this tool would benefit from understanding the tool is designed to make a string a fillets that connect two sets of tangent surfaces. Those two sets do not have to be joined together or even touch each other. People who set things up as if they were going to use filletedge are going to miss out on what this tool can do
Throwing non-tangent surfaces into the mix is just going to throw a wrench in the works.
This file shows how you should be thinking of this to get the most from the “continue across faces” option in Filletsrf. After you make the first set of fillets you again have another two sets of tangent surfaces that can be connected by fillets.
designed_to_succeed.3dm (248.1 KB)
If you make the second set of fillets the same size as the first set of fillets with the current implementation of “continue across faces” it fails because it can’t handle same size fillets properly. Thats is a bug.