>>529080019
That's what I found when I searched for it. I suppose it was a one in a billion as I've been using that Filth Washes Away with Rain and Time mod for ages and I've never had this occur.
Perhaps the chance was heightened because it's a dirt-able tile right between two grow zones on dirt tiles, born from how I built these greenhouses which are different from what I normally do.
Using dev mode Destroy on the -5200 tile and the one Reika was standing on which also had a negative dirt value released her from purgatory.