Code dive as requested, there are three entries in data/json/itemgroups/main.json and one in data/json/item_groups.json
The three in main.json are adding them to the rare items list, the entry in item_groups.json adds the basic one to the military items list.
The vault item group has all three power armor types, but no helmets, that’s a bug.
The mil_armor and mil_armor_torso groups have power armor, but no helmets, probably a bug.
Diving a level down, let’s look at here these spawn.
“rare” is spawned in:
The “military fight scene” special with a bunch of military corpses.
The “science group scene”, same as above.
Wasp houses (If things spawn on the sidewalk, that’s a “bug”).
Spider basements (both varieties)
Survivor cave
Rat cave
'military" spawns in… whoops that’s a spoiler:
Also in helicopter crashes.
As for groups where armor currently spawns without helmets:
mil_armor spawns in:
Armory of the military outpost
Supplydrop special.
Bio operator drops.
Vault list spawns in the vault, go figure.