The UO Test Center 1 has been updated with some anti-leeching measures limiting the amount of drop credits gained by AFK and hidden players inside dungeons. We will hopefully see these changes hitting the operational game servers soon.

Ultima Online Test Center 1 Patch notes
Test Center 1 has been published with the following:

• You will get less credit towards the new dungeon drops if you are hidden when the monster you are fighting dies.
• You will get no credit if you are more than 16 tiles away from the monster when it dies.