I don't use the DAB as I find it a pain in the a$$ (and use Spotify via bluetooth all the time). However, I've had the volume reset to zero a lot of times. My dealer has discussed this with Mazda who apparently have "never heard of it happening".
When it happens (sometimes mid song) it's not just volume that resets to zero, the bass, treble and balance controls all reset to default positions as well. MZD software was updated on Friday, time will tell if it still occurs.
Alan