Second thing: eight centimetres is nowhere near deep enough for a bull millipede. They moult underground in a little chamber and they need to bury themselves completely, which for the larger species means several times their body length.
And the moisture target is damp like a wrung-out sponge, with a gradient so one side is wetter than the other and they can choose. Misting every couple of days wets the surface; what you want is substrate damp through.