I deployed with an infinite durability supressor that I didn't research. Why?

MGS 5: TPP On Xbox One

My normal loadout consists of a Renkov Tranq Conversion sniper rifle. I got the legendary gunsmith, and customised this to add a grade 1 suppressor rather than researching the suppressor for this rifle (also a neat paintjob).

It works great - however something really unexpected happened the last time I deployed. My suppressor on this rifle had infinite durability, marked by a small infinity sign next to the suppressor bar on the weapon when equipped. After using several magazines of tranq rounds, the durability didn't go down.

I was about to start story mission 19, and was just running around stealing resource containers and capturing guard posts in Africa. I started at the oil well base, and moved south - east from there, capturing everything in my path, all the way to the mine, picking up everything I could - I wonder if this was an item I picked up and didn't notice.

I called in a supply drop multiple times and kept my infinite suppressor, but mistakenly called in a loadout (containing the same weapon) and lost it when I jumped in the box.

TL;DR

Where did my infinite suppressor come from - any chance I picked it up as an item while running through the various checkpoints and bases I was raiding? Is it a bug, exploit, or is there a random chance I will get one?