Ifixit replacement screen problem with autobrightness

A possible fix is going into Settings < Accessibility < Display Accommodations < Turn Auto Brightness Off

Then go back to Settings < Display & Brightness < Turn Brightness all the way down

Go back to Settings < Accessibility < Display Accommodations < Turn Auto Brightness On

It helps to do this in a dark room. If this fix doesn't work, it could be a loose pin on an individual cable, or a short in the board during the repair.

I also had this problem (along with my phone screen not going black during a phone call when the phone was up to my ear) -- I ended up reinstalling the original camera and sensor from my Apple screen onto the replacement screen, but the problem kept happening. When I opened the screen back up and looked at the connection where the ribbon from the sensor connects to the base portion of the iPhone, I noticed one of the gaskets looked like it was folded over into where the connection is made. I cleaned this area with some rubbing alcohol, readjusted the gasket, was careful to move it out of the way during reconnection, and now the Auto Brightness is working again. Hope this helps.