From 1.20 notes:
[dev1273] A single gathering attempt will now yield no more than one of the following items, as opposed to the previous maximum of three.

Cobalt Ore / Electrum Ore / Carbonized Matter / Petrified Matter / Fossilized Matter / Crystallized Matter / Germinated Matter / Decayed Matter / Decomposed Matter / Liquefied Matter / Calcified Matter / Cultured Matter / Ossified Matter / Cretified Matter
I'm out gathering in Treespeak for Mythril Ore and can't seem to dig up more than one of those at a time. Was mythril added to this list during a later patch? If not, could this be a bug?