Javelin bag both can't hold javelins (they're too long for it) and it's encumbrance when filled (with the 6 theoretical javelins) is the same as a full dufflebag which seems to be wrong
Spawn 10 javelins of each type (wood, metal) and a javelin bag
try to put the javelins in the bag through either Reload, Insert, or Advanced Inventory
Javelin bags should be able to hold the one thing they're designed to hold
Javelin quivers should be nowhere near 35 torso encumbrance based on historical designs
Encumberance

Javelin length is too big for bag (also volumes don't match pockets)



Can't insert Javelins

Can't pickup javelins with bag equipped

Looks like it's an issue with length, though the encumbrance needs to be re-balanced and the pocket volumes should probably be updated but unless it confuses the encumbrance check that's probably minor
Can confirm that javelin_bag is indeed missing max_item_length entries in its pockets. Some other specialized containers also seem to have the same issue. This happens when an item is given a length but the associated container isn't updated accordingly.
Do you have a link to the historical javelin bags you mention preferably with pictures of how they are worn?
a good alternative would be to allow small shields to store up to 3 javelins as they were mostly used by skirmishers in combination. atleast thats a good way until you are able to hold things in your left hand aswell.