FWIW, I’ve been having this issue since 11.2 or so and every single release afterwards. The only thing that fixes it is switching to iPhone speakers and then back to Airpods as the output, which is a temporary fix really. Is it the same for you?
I’ve had Airpods replaced by Apple and unpair/re-paired them a few times, however, the issue still persists.
Hoping iOS12 fixes this.