I will venture a guess that you have two separate issues.
The missing piece is affecting your metering, as my guess is the diffused section of the missing piece 'smooths' the light for the meter into one light. Without it, I think the meter may be getting confused by not seeing one light level, and seeing areas of dark and light.
The yellow overlay is probably out of alignment, and needs adjustment to be calibrated properly. Whether or not that's a DIY on that camera, I don't know.
What happened to the missing piece? How did it get lost/removed? My thought is there was some impact that caused it to come off, as well as knocking the RF out of alignment.