When I set it, and then lock the iPad, often it doesn't save the changes. Like the screen will still be dark when I unlock it, and as soon as I tap the slider (even if I don't move it) it fixes...annoying.

nope

do you have the auto brightness on?

Nope....

Yes, I've seen this a few times.

It's because it's just changing the brightness in SBSettings and not saving it in the actual brightness and settings for it.

Here, try this: Go into Settings and turn the brightness all the way up then press the Home button then swipe across the status bar so that SBSettings pops up. After it pops up, go to brightness and then moventhe slider all the way to the left so your screen brightness is as low is it can possibly go. Now, go into Settting and look at your brightness - it will show that the slider is all the way to the right as if your brightness is at max but it's really at the lowest setting because you changed it in SBSettings, but it didn't save it in Settings.

Nope even in the settings it's maxed out on bright, and yet it's still showing dark. It's simply not working once it gets locked.