Even more odd. The regen rate is adjustable. It will drop to almost nothing if you are on icy roads. I even see people mentioning it happens on dirt roads sometimes. Basically anything where you have less traction.
On some Tesla's you can even press on the accelerator lightly, but don't engage the motor, and it will reduce the regen rate. People do this going down hills. Seems like a lot of work to squeeze out an extra 10 miles on a charge.