I find it drops close to that of OSI. Perhaps you have been unlucky. The one issue there might be is who actually drops them.
As I have received several off of Master Jonath, however, he doesn't appear on this list:
*Non-Paragons that drop chests
This afternoon I'll thumb through the scripts to see if the Mobs on this list [*] are all dropping them and petition for Jonath to be corrected.
Source:
https://uo.com/wiki/ultima-online-wiki/gameplay/treasure-maps/
Paragon Ancient Wyrm, Paragon Balron, Paragon Betrayer, Paragon Blood Elemental, Paragon Ethereal Warrior, Paragon Greater Dragon, Paragon Meer Eternal, Paragon Poison Elemental, Paragon Rotting Corpse, Paragon Skeletal Dragon, Paragon Succubus, Paragon Titan,
*Miasma, Pyre, Rend, Abcess, Putrefier, Master Theophilus, Stygian Dragon