When the car was on the dyno was it fitted with egt sensor in each header pipe. This will show the reason for the power dip if its reversion.
Try a one and a two inch or large carb spacer and run it again. More volume will minimize the problem if the intake is starving for a/f mix.
But like others have said its real common for cars on a chassis dyno to slip right at the top of the torque peak as the power comes on. When your dyno guy seen this graph did he tigthen the car down and run it again or say here you go and start unhooking chit. I would think that flat graph right around 5252 rpm deserved another look.
Also are your tires capable of putting 500 hp to the ground without slipping. Wide enough and had the right tire pressure. Another thing that should have been adjusted and retested after this run.
I like to put the cars on jack stands and check for drive line drag. If its rolls smooth and quiet its probably not your problem. If it takes two hands to trun the tires over then good place to start.
Hope this helps.