Per the dev notes, changelog, and sarbian's thread about how to update mods for 1.1, modules can no longer set part.mass direclty; instead part.mass is set every tick via GetModuleMass calls. This means that the GetModuleMass method must not reference part.mass directly, amongst other things. It needs to report the delta between the prefab's mass and the desired mass.
Per the dev notes, changelog, and sarbian's thread about how to update mods for 1.1, modules can no longer set part.mass direclty; instead part.mass is set every tick via GetModuleMass calls. This means that the GetModuleMass method must not reference part.mass directly, amongst other things. It needs to report the delta between the prefab's mass and the desired mass.