iphone 15 pro max battery health drainage after ios26

Force restart your iPhone:

Have you tried a forced restart:

1. Press and quickly release the volume up button.
2. Press and quickly release the volume down button.
3. Press and hold the side button until you see the Apple logo (this might take longer than 10 seconds).

The operative word here is QUICKLY for the first two buttons, then a long hold for the SIDE button; and try it a couple of times, the timing is a little tricky. If done correctly this will always work unless the phone has a hardware problem. 

Also, from→About Apple software updates - Apple Support:

Immediately after completing an update, particularly a major release, you might notice a temporary impact on battery life and thermal performance. This is normal, as your device needs time to complete the setup process in the background, including indexing data and files for search, downloading new assets, and updating apps. 

You will need to give it 3-4 days to finish background indexing of your files and optimizing your photo library for object detection algorithms. If you have a large photo library, give it another day or two. That is normal after every major update and to a lesser extent with point updates and seen here in the Apple Support article to expect battery drain, lagging, and the device will be warmer than usual:

About Apple software updates - Apple Support

Yes, TikTok does use quite a bit of your battery and if you see Background Activity noted at Setting > Battery for TikTok, then it is using the battery even when you are not using the app. If you are still having files indexed in the background, the battery drain is even more noticeable.