Have seen it many times. When creatures, including workers, go somewhere, they don't care much about avoiding damage from tiles. Even if there is a clearly safe path nearby, they often prefer to go near lava/melted gold tiles, close enough to lose health. If you have workers mining golden shrine near lava, they will move near it all the time to carry mined gold to your dungeon, and eventually will die.
For comparison, in dungeon keeper creatures actively avoided lava. Dunno if it's a bug or a feature to encourage players to build more stone bridges though :)
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