Yep; cost effectiveness of a Ballistic Mask + regular Helmet (non-Full version) is just too good.
Cannot take Chemical Seal:
Ballistic Mask (+2 AV, 8 Capacity, Avail 6, 150¥)
Urban Explorer Helmet (+2 AV, 6 Capacity, Avail -, 100¥); regular helmet is Avail 2 for some reason
Can take Chemical Seal:
Full Helmet (from core; +3 AV, 6 Capacity, Avail -, 500¥); availability is weird, maybe it has to be bought with the suit?
Hardned Mil-Spec Battle Helmet (from R&G; +3 AV, Capacity 8, Avail 8F, 10,000¥)
Allowing the helmet to stack with the ballistic mask makes it too good of a combo for a measly 250¥; a whopping 14 Capacity with low avail and low price for +4 AV is hardly game breaking, but I think it does mess up the balance of the Full Helmet, which at that point only has the Chemical Seal option (costing 6000¥ for the seal and at least another 2000¥ for the Full Body Armor, I might add) going for it.