Trial of the Sekhema / Floor 2 Hourglass Trial / Bug Report

There seems to be a bug with the Hourglass trials on floor 2.
Whenever the room does not spawn with 3 of the Rare Vase-monsters in the room there is a chance rares will either not spawn altogether or suddenly stop spawning.
This is can make you wait around up to the full 1:30 minutes before you can move on.

To me it seems like the room will attempt to spawn a Rare mob, but if the non-existent 3rd Vase mob is picked the room will bug out and stop spawning rares.