I was post mission, reloading guns with new mags and leveling up my heavy then suddenly after I clicked to the geoscape, I went back and all his gear was sniper armor and a rifle and pistol and all of his equipment was gone. Now I have a spare sniper loadout but have to rebuild the heavy gear.
Not sure on the specific trigger, all I can offer is after a level up (level 6)
this is a bug that was supposed have been squished, but somehow made it back in
I have had this happen as well, both times where with a Assault(primary)Sniper(2nd) class and both times they morphed into a full set of Technician gear. once was just changing gear around and noticed it the the next time I went back into level them up from training at the base. next time was after i had sent a team out on a mission and when they arrived one of my units had morphed all his gear.