Display Brightness won't go over 50% after chassis swap

So this is really confusing me, I've rebuilt quiet a few iPhones over the last couple of months including bare metal rebuilds.

I'm faced with a problem I can't work out, I have this one after Market chassis, no matter which logic board I put in the chassis the screen brightness seems to change upto around 50% on the slider but the last 50% screen doesn't get any brighter, i figured problem with backlight circuit on that logic board, but when I fit the same logic Board and screen to another chassis then it all works fine. In addition another logic board tried in same chassis same problem...

What could it be?

I'm not swapping the dock connectors or power switches out of the chassis could maybe a faulty power switch or dock connector cause the brightness of the display not to exceed 50%?

This one has be stumped, maybe a ground problem with the after market chassis?

Any ideas?

My next step is to swap out the dock connectors and the off on switches to see if this could be the cause....

Thanks.