I've tested it on several archeotech weapons and it's on 99% a bug. I've made:

10x Archeotech Heavy Flamer with Archotech(upgraded) skill

5x Archeotech Autogun with Archotech(upgraded) skill

5x Archeotech Shotgun with Archotech(upgraded) skill

5x Archeotech Inferno Pistol with Archotech(upgraded) skill

They all came with 1+3 enchants slots. And it's not a bad luck or something. 

Just to confirm it I created normal Archetech Weapons:

3x Archeotech Autogun - each rolled with 4-5 enchants

Please fix it as soon as possible as this is a major bug for the endgame. I literally wasted  hard to get crafting blueprints to get bugged item (Obviously I'm aware of it that no one will compensate it for me and make it a 5 slot weapon as I would normally get it with that amount of crafting)