iPhone 17 Pro Max battery percentage not updating after iOS 26.1 update

I'm not sure if anything is really wrong.

If you take the phone in to the Genius Bar or authorized Apple service shop, the first thing that the tech will do is perform a Forced Restart. (This is different from simply restarting the phone.)

If you have not already tried this, it might save you a trip.

Force restart iPhone - Apple Support

Please follow the instructions very closely, as users often do not.

Quickly press and release the Up Button

Quickly press and release the Down Button

Hold the Side Button in for at least 30 seconds or more until you see the Apple icon appear in the center of the screen, then release the Side Button

(When you hold the Side Button in for a few seconds or more the "slider" will appear. You have to ignore that and keep holding the Side Button another 15-20 seconds or more until the Apple icon appears, then release the Side Button)

It's probably going to be another 6-8 weeks or more before the next update, and there is no assurance that the thing that you are seeing will be addressed. Occasionally, I see this on my phone, but I don't really pay attention to it.

Good luck

I own iPhone 17 Pro Max and sometimes, if I'm not doing much on my phone when I get up in the morning and the battery is at 100%, it can be a few hours before the battery shows any drop. And when you restart the phone, the calculation usually will show a decline. Based on what you're describing, I don't see you have anything wrong with your iPhone. The 17 Pro Max has the biggest battery ever put in an iPhone. It will last longer than you think it might and yes, even remain at 100% longer than you thought.