iPhoneX auto unmute

Throughout most of each day I choose to have the "Mute" function on - NOT the hardware button, but using the "Device" -> "Mute"/"Un-Mute" (bell) software toggle: I am simply not a fan of a phone that keeps "ding"-ing and "bong"-ing and trying to capture my attention at random times. However, I make extensive use of alarms and timers, so do want those to continue to work during "Mute", which they do. I also use the Do Not Disturb function during sleep hours, and this also works well.

However, the phone decides to Un-mute itself nearly every day now - I can only assume that it is happening at the end of Do Not Disturb time. This is NOT how I want it to work, and it is NOT how it worked prior to a few updates ago (11.2.x). The state of Muting should be preserved after Do Not Disturb.

This condition used to occur sporadically for iOS 11.0 and 11.1 updates on my old iPhone 6S, so it doesn't really seem to be a device problem. It used to resolve itself by a reboot, but that doesn't seem to work any more.

Is there any other setting I should be using, or is anyone else noticing the same thing? I have tried searching for other incidences, but I have seen innumerable hits for "phone mutes itself", but that is exactly the opposite of what I am searching for.

I do not have the Bedtime hours set, so that's not it either.