This happens with all Eris's.
Even ones rooted, with custom ROMs, based off of the original Eris Software.
Since the July OTA update, I've noticed this too.
If I'm using Camera, and I go 'home' then 'End Call' (to turn the screen off) too quickly, the brightness is SET at 100% by the time I turn the phone back on.
I have to put it back to Auto.
There isn't a good fix for this, except WAIT. If you use the camera, then go 'home' then turn the screen off, WAIT for the brightness to kick back down from 100% first.
Same with any flashlight app.
Same with a lot of SMS popup apps. They'll get it stuck at 100% also.
No good fix unless you root and use like a Froyo or Sprint-based ROM or ROM based on the older (less updated) Eris stuff (like ivan's eris_official).