Ruin Explorer
Legend
The big problem here is, the Druid that does that has essentially left his allies to do die. And they will die. The Druid's spell might eventually kill or do a lot of damage to the enemy Fighter or Paladin, but combat typically lasts three rounds.Disadvantage on all attacks on an enemy warrior, plus 90ish damage over 10 rounds, is great value for a 2nd level spell, especially as you level up and the threat a low level spellcaster poses goes down. A CR 2 Druid has 11 AC and 27 hp, they're better off running and getting all that then sticking around and getting murked by any player who looks their way.
So the enemies will all be dead by the end of round three, usually. And they'll die faster and easier without the Druid there to cast other spells or cause a distraction.
You have to find a situation where it makes sense, RP-wise, to have a Druid who runs and leaves his allies to die, too. As literally the first thing they do in the fight. To call that "corner-case" is generous. If you DM isn't a hardcore ambush-griefer like poor Mistwell is dealing with (and I do feel for him, that is excessive ambushing, I hope his party is built to deal with it), this might happen one session in one hundred. That's rarer than corner-case.
On top of all that, your DM has to change the statblock for the Druid to intentionally put in Heat Metal with the intention of using this quasi-exploit/abuse against his players. That speaks poorly of the DM who does it.