Automatic Dark Mode turns off before sunrise

Worth checking that Location Services is on, and under Privacy & Security > Location Services > System Services that Setting Time Zone is on too. Sunrise/sunset for Appearance comes from your location, and it can differ a bit from what Weather shows.

I have not encountered the behaviour that you describe.

Ensure that you have updated your iPad to the most recent version of iPadOS. At the time of writing, iPadOS 26.7.2 and iPadOS 27.2 are current:

https://support.apple.com/guide/ipad/ipad9a74c576/ipados

If your iPad is not operating as expected, try a forced-restart - as this will often clear an isolated or post-update software issue:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

If the problem persists after performing the restart, it may be necessary to reset your iPad settings. While this reset will not cause any loss of data, as a precaution, it is always wise to ensure that you have a current iCloud or iTunes backup before performing the reset:

How to back up your iPhone, iPad, and iPod touch - Apple Support

When you are happy to proceed, you can perform the reset from iPad settings:

Settings > General > Transfer or Reset iPad > Reset > Reset All Settings

If the problem still doesn’t clear, the next step is to reset the iPad to factory settings:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

Otherwise, you would be best advised to seek assistance directly from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:

https://apps.apple.com/app/apple-support/id1130498044

You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to assist with troubleshooting:

Genius Bar Reservation and Apple Support Options - Apple

Turns out all my devices - iPhone, MacBook Air, and iPad mini - switch to dark mode at exactly sunset but switch to light mode exactly 20 minutes before sunrise.