Thank you all for your comments. Here's an illustrated version to make things clearer:

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.

P13 creates .pz2 and a .pmd files as seen on the folder on Windows Explorer.

Then on P12 I load V4 and go to the poses library...

...and load "Horn". It works.

Back on P13 I delete the V4 and load a new V4 and then load the Horn pose...

...and while the dial appears it has zero effect on the figure.

It's not that P12 injections for V4 don't work on P13, injections created on P13 also don't work. They all work fine on P11.

And when I try the same with M4 I find no problem at all.

I keep thinking there's something wrong with my setup because otherwise a lot of other people should have had this problem. I guess I'll have to submit a ticket.