Based on the install instructions (
All Pro Off Road) - the OE mounts are used. The only possibly relation to your problem and the All-Pro bumper (that I see) is the installer did not allow enough gap between the bumper and OE sheetmetal, or perhaps you did not retorque the mounting bolts, and the bumper is chaffing against OE sheetmetal at that location. Once the paint and primer wore away, then the metals began rusting via oxidation.
Step 13 could also present a similar problem if overlooked during installation.
"13. Dry fit the bumper onto the FJ frame end rail studs. You may need to adjust the two sheet metal brackets underneath the headlights to allow the bumper to fit over them without interference. There is a small tab that sticks up from these brackets towards the headlights. Bend these tabs downward using a pair of pliers so that they are flush with the rest of the bracket. After confirmation of fit and clearance."
