Auto-lock malfunctioning

A forced restart may be helpful - as this will often resolve an isolated software issue:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

Do you have a work email or Exchange account on your device? If so, the employer who owns the Exchange account may have set a policy to prevent those changes which would be installed on your device when you added the mail account.

Make sure that you do not have any restrictions active in Screen Time to restrict any system settings.

Have you configured both the iPad Passcode and TouchID/FaceID?

Settings -> Display & Brightness -> Auto-Lock

However, we’d urge extreme caution about setting it to “Never.”