>>>> On P13 I load V4 from an outside runtime and using a magnet I create a chest MT I'll call "horn". Then export it as a morph injection to the poses folder of that same runtime.

This might be the problem. You're creating the morph with a magnet, and then export as a morph injection. 

What I would do first is Spawn a full body morph for the figure. This should take the shape from the magnets and create an actual morph so that you don't need the magnets any longer. Just spawn the morph, delete the magnet, dial the morph to 1 to make sure it works as expected. Then create your morph injection and PMD. Let me know if that fixes the issue.