Did it just friction break or did the ICE turn on to provide engine breaking? Pure friction breaking can be unsafe on long steep declines.
I'm pretty sure the engine didn't come on (nor did the display drop from 4 to 3 chevrons like at high speed). There was still a significant regen braking feel, and I didn't have to ride the brake pedal the whole way down, just occasionally to slow for each curve and on a few very steep straight sections.