I have heard they can burrow but i have never seen mine burrow in cypress mulch.
The mat operates at a max temperature of 100f and theres like 2-3" of substrate inbetween the snake, and my rheostat shuts off power when the temperature reaches 90-93f ~.25" under the surface. The max temps of the ground in his hide this week ranged from 90-93f, im not worried about burning my snake.
Unless he decides to burrow all the sudden.