Equipping Stampede on your character breaks your Mercenary's movement speed

Bug Report ID: #2550999054

Basically what the title says. I recruited a new Bastion mercenary on a character wearing Stampede boos. I noticed he was horribly sluggish and couldn't keep up with me even though he had a well supported Shield Charge. I then checked his stats and realized that his Movement Speed was -6%, despite having a pair of boots with 30% MS.

I thought that maybe this was something inherent to that class of mercenary, but when I switched characters to one wearing random rare boots, his MS was now displaying +24%. So I checked if swapping from Stampede on the first character would instantly fix his movement speed, and sure enough it did.

Here's a quick video of it breaking: https://www.youtube.com/watch?v=grkxfZGmzTY&feature=youtu.be

If I had to guess, Stampede has some hidden stats that multiplies all other MS modifiers (except the innate -6% from body armour) by 0, which for some reason gets inherited by the mercenary who doesn't even wear it.