I disabled auto-unlocking DLC weapons in V1.9.8 and made them buyable in the store instead. But while DLC weapons stored along with vanilla ones (like 'Merica) worked properly, all weapons stored in DLC archives (dlc1.vpp_pc, dlc2.vpp_pc, etc.) would become locked every time you load the game for some reason. It seems that the game wouldn't save info whether you own them or not and they were simply auto-unlocked while loading the save... Apparently this has been fixed in the modding patch because I didn't experience this issue unless I eventually uninstalled it.
-Iker-, Spanish translator, reported it few days ago and
Singa, German translator, confirmed it later, we spent more than an hour to find out the reason (thank you so much, guys!). I fixed it by simply getting rid of buying in the store feature so they're now unlocked automatically after 'The Real World'... There's no other way, unfortunately.
DLC weapons stored along with vanilla ones still have this feature though so these ones aren't unlocked automatically.