Outdated:
"They drop to the corpse of the monster killed"
Where did you see this? I can track down this quote on the ML artifact page, ("Alternatively, Peerless bosses will sometimes drop these artifacts on their corpses.") but not really sure where you saw your quote. Plus it specifically says peerless, which Miasma isn't one of. Just trying to find out what you're seeing that I'm not.
There was this quote, which appears to answer your question, but I'm just guessing. "The
[bcolor=rgb(16, 16, 16)]Mondain's Legacy[/bcolor] Minor
[bcolor=rgb(16, 16, 16)]Artifacts[/bcolor] are 21 magic items that were introduced to gameplay on August 30, 2005.
They are randomly earned when defeating
Named Monsters that reside in the nine
Mondain's Legacy Dungeons, and appear directly in a player character's
backpack.
Sentinel Spider inside the
Underworld also drops these artifacts."
As for you spending days on Gnaw, RNG is gonna RNG.
It's funny, the "official" wiki directly contradicts your experience.
Artifacts – Item Sets – Ultima Online
So, either continue to use uoguide, which is outdated but still largely accurate, make your own wiki and find people to help, try to get the official one to be accurate, or insult people trying to help. Your choice.