Not enough mobs spawing to fill/complete the the pestilence bar in Urban glades sometimes. Even if the full dungeon below is cleared proably. Please check.
Maybe a combined problem if at the same time a bounty is active. In my the bounty case “Forgotten Truth” is active as well and cannot be completed because pestilence is active in this area.
So my personal solution was: Doing something else in another region and coming back later and kill the respawn in this area to spawn the boss.