This is a quite interesting behavior.
I don’t know if that is connected with your issues, but I also experienced some strange behavior with the volume button.
I was pretty much struggling for a week now to use the volume down button, it just hardly responded. I hardly ever could make screenshots, and also the volume setting didn’t respond. It seemed though that if I was first pressing volume up, then volume down also more likely reacted again.
Today I rebooted the phone a couple of times (usually I don’t turn it off, just in flightmode), and now it works absolutely fine again.
I was close in writing the support team a message that I think there might be an hardware issue with the button (well, maybe there is after all). But for now it seems to work fine again.
I will keep an eye on it. I also don’t know how to reproduce the issue …