In the terrain's settings, when you set the Terrain Height to a high enough number, Thermal Erosion breaks and starts to become almost an set-height-to-zero tool.

Repro:

Have a terrain whose Width, Length, & Height are 500, 500, 500.
Have some rough mountains on the terrain.
Use the Thermal Erosion Tool.
Works

Undo.
Change the Width, Length, & Height are 5000, 5000, 5000.
Use Thermal Erosion again.
Breaks
It's only the Height that matters, but these repro steps show that the ratio doesn't matter.

Expected:
That the Terrain Tools work regardless of the terrain scaling, since it has nothing to do with resolution.

See attached GIF