I added the backpack mod on the companion armor and it's not increasing the total weight by 20. I can swap the armor to the main character and its correct, but when I move the armor back to the companion it no longer increases total weight. 

I just run into the same issue.

It was working first time I modded the armor. I saw 110 max weight and now its just 90 and has no effect on companion armor.

Hi, I just found this bug tonight.

Please resolve when you can, either by adding the proper bonus to Carry Weight or by adding a note somewhere in the game information that explains that the bonus will not apply if used on a companion. IMO Ideally it should warn you when installing the Modification, since this is a semi-permanent change to your gear.

I have included screens of the issue.

 

I apologize that you've all run into this bug and for any frustration it has caused you during your playthrough. If you haven't already, I recommend submitting a bug report to our friends at Private Division as they are the dedicated support for The Outer Worlds.