iPhone notifications - `volume: x.x` has no effect?

From the docs (as I have interpreted them), the following should send a loud tone provided the phone is not on silent and the sound in question has been imported:

action: notify.mobile_app_monkeybutler
data:
  title: "test"
  message: "test"
  data:
    push:
      interruption-level: time-sensitive
      sound:
        name: "Bloom.caf"
        volume: 1.0

However, that volume command has no bearing on the actual volume of the notification; I can set it to 0.0, 0.5 or 1.0 or even remove it and the volume remains the same.

Also, if I add critical: 1, I get full volume whether volume: is there or not so, the following two also yield the same result:

action: notify.mobile_app_monkeybutler
data:
  title: "test"
  message: "test"
  data:
    push:
      interruption-level: time-sensitive
      sound:
        name: "Bloom.caf"
        critical: 1
action: notify.mobile_app_monkeybutler
data:
  title: "test"
  message: "test"
  data:
    push:
      interruption-level: time-sensitive
      sound:
        name: "Bloom.caf"
        volume: 1.0
        critical: 1

i.e. (for me) volume does not work at all. I need to respect the phone’s ‘do not disturb’ setting, hence using interruption-level: time-sensitive not interruption-level: critical but I also need the volume loud (this is for my mum).

Is this the same for others or can anyone advise me?