The same thing happens with brightness too. Turn it up full, suddenly, down to 3/4's again until you acknowledge yes, you really do actually want to turn your brightness up full.
The first time it happened I was outside in the sun and couldn't actually see my screen to figure out why it dimmed again, i kept trying to brighten it and it kept dimming, I finally noticed the popup on the bottom in the brief second of lightness I had after the 4th or 5th try.
I really hate the way devices try to control you rather than allowing you to control them.
On my old rooted android phone from years ago, I had alsamixer installed and could turn the volume up however I pleased, even above the artificial software limits present in all android versions.
I miss those days.