I had that fault quite frequently when I first got the car, but much less often recently - only a couple of times in the last couple of months. One time it was raining and I wasn't in the mood to mess about so I just drove home without the auto-hold, keeping my foot on the brake pedal instead. The most recent occasion I was about to leave the house, but just got out of the car and back in again and the fault was gone.
There have been various theories about it being caused by depressing the brake too soon after getting into the car, before the infotainment screen has booted up, but there's no rhyme nor reason to it. On the most recent occasion I simply sat down in the car, didn't touch anything, and the auto-hold fault appeared.
It's a bit of a nothing-burger, and I'm reluctant to mention it to the dealer. There may be a fix with updated software, but since everything else is working, I'm thinking this isn't broke enough to risk "fixing" it.