Erratic iOS battery meter and usage from Music app randomly playing on mute Since 3 weeks ago the battery meter on my iPhone 6s with iOS 11.4 has been highly inaccurate. The meter will stick at a certain percentage far longer than it should, then rapidly drop down to a lower level, stay there, and then drop down again. For example: it will stick at 100% for a few minutes on end, plunge to 95% over half a minute, stay there for 8-10 minutes, then plunge again to 90% within another half minute.

The erratic battery meter behavior doesn't coincide with an OS upgrade (11.4 had been fine for months leading up to this).

iOS Battery Health doesn't report any problems with the battery - it holds 92% maximum of what it originally held and is considered to be in "good health".

I previously blamed and associated this behavior with a specific mobile game that was updated recently. However, I observed the battery meter plunges outside of that game just recently (it occurred on the home screen) so it's been ruled out as a cause.

Additionally, the built-in Music app is randomly waking up and silently putting itself on play over speakers with volume on minimum or mute, wasting 20-30% battery over several hours. This happened yesterday at 1-2 PM estimated and again overnight today.

When I first checked for excessive power usage, the game I mentioned earlier used off-the-charts amount of power for "audio". Now with the built-in Music app acting possessed and waking up with no user input, I'm inclined to blame the iOS audio system.

I already read the previous question on 
Erratic battery percentage behavior which doesn't fit as it asks about a jailbroken iPod Touch.
According to the battery meter, your battery appears fine. However, the symptoms you describe are typical of a bad or dying battery. Ideally, you would want to replace the battery with a known-good battery (from another device, for example) to see if the problem persists or not.

In the absence of a known-good battery, one purchased from a reputable supplier can do.