Sound still plays when phone is completely muted

When using fairly common apps like Instagram and Garageband, quite often I will lose control of the volume and it will be permanently on the highest setting, regardless of whether I turn it down or even mute it. It seems like the volume controls are being completely overridden by something; this is not very helpful at night when others are trying to sleep.

On Instagram, I can close out of the app and open it up again and it will be fixed, but Garageband does not work in the same way - I've had issues with Garageband before, but I'd assumed updating to the latest iOS would fix that.