Okay it looks like my previous fix was a little too naive and it broke a few rocket launchers in the process.
The root issue here is that the FF2 and VSH plugins applied an item attribute to the mantreads that gave them a damage force boost for rocket jumping. Unfortunately it looks like Valve has stopped this attribute from working on non-active weapons.
What I'm going to be doing is moving this effect from the mantreads over to the rocket jumper. The rocket jumper will now just be a reskinned regular launcher that allows you to rocket jump further than the others.