The bug is positive, but I don't think it's intended. I've noticed when I buy Neutroamine from a village or caravan, I actually gain capacity instead of losing it in my caravan.
You forget one important things.
Silver the currency isn't weightless.
Neutroamine is just got a higher market value and weight less compared to silver.
That happen to many other things, scyther blades, thrumbo horns, medicin, high quality weapon/armor.