I was trying to setup my crappy Gambit gear from my bank, and noticed I can't install the season's grenade or melee shield-piercing round mods. No issues with the Overload Grenade mod. Tried it with the exotic chest I found recently, and same thing. Luckily, I tried equipping the chest armors, and the mods stopped saying "similar mod installed" even though no mods were even installed. Easy work around: just equip the armor you want to respect. I was just really worried I wasn't gonna be able to respect my high stat gear. As far as I can tell, it's just the season chests mods doing. No issue on any other armor.