If I'm understanding correctly, you're not worried about the tortoise burying himself, but the layer of substrate that's still on him after he un-buries. That's why I didn't like coir - well, that and the strings. If you use coir, you have to expect that when he comes out he's going to still have some stuck all over him. It's not harmful.
Yeah a lot of tortoises and turtles like to dig and inevitably get dirty. Sometimes they will rinse it off on their own in a shallow dish but most times they don’t care about dirt like that since in the wild all it takes is a bit of rain to get all clean again. During feeding time one day I plopped down the plates of food and came back a second later to see my three toed box turtles eating and one apparently buried himself pretty deep since he still had 4” of dirt stacked on his shell lol