I think the sump plugs are brass.
Mine is, and the basic form is cast/forged ... including the hex socket, which is actually slightly tapered to help its release from the "mould".
Therefore an allen key will only make full contact at its very end, and hence the hex socket gets rounded off quite easily if you're just the slightest bit careless with it.
Mine's fine, obviously
..... but I had to grind a hex key to the same taper to get the sump plug out of He11cat's engine.