Rockomax Engine and Decoupler contracts make the experimental parts available without the required adapters and fuel tanks.
When accepting the rockomax skipper/poodle engine or decoupler contracts it might be hard to test the parts without having the appropriately size fuel tanks and adapters.
The parts connect without an adapter: command moule -> RM-decoupler -> regular-size-tank -> big RM-engine, but it looks weird of course.
A rocket built ths way flies normally though.
You decide if this is a bug or not. :)