I used my only healer mech serum on my newly recruited melee guy to heal his torso scar, but the game somehow considered the +15% movement speed from his monosword to be more important to "heal".
I generally don't like to do this, but I felt perfectly justified to use dev mode to heal the scar afterwards, now I know that you should unequip them first, and so do you.
Getting the +15% movement speed back was simple, I just had him unequip then reequip the weapon.
It's a bug. Sorry about that - we'll fix it.