Q: Screen brightness suddenly during use is lowered while it's the highest in settings "I'm turning off the auto brightness" and it doesn't come back to it's highest brightness untill I restart the device, what's the problem and how to solve?

Hello there, M7mad Mousa.

It seems you are seeing an issue with your iPad's display dimming, although you've set auto brightness off and the brightness setting is set to it's maximum.  The following Knowledge Base article offers up some great additional steps for troubleshooting:

Get help with the screen on your iPhone, iPad, or iPod touch - Apple Support

If your screen is too dim, too bright, or you see only faint images

1. Try to adjust the brightness. If the screen doesn’t respond when you adjust the brightness, open several apps to make sure it’s not an issue with only one app.
2. If you have a screen protector, try removing it. Try removing your case if it includes a screen protector.
3. If you see the issue in multiple apps, restart your device.
4. If restarting doesn’t fix the issue, restore your device.
5. If you still see the issue after you restore, contact Apple Support.

Thanks for reaching out to Apple Support Communities.

Kind Regards.