Thanks for this. I think you are correct. My Z axis was near the limit of its travel and the steel used for the part is gauge plate.
The part is pretty much similar on both ends. The machine didn't seem to have a problem ramping the holes out. The mark is where the end mill first makes contact to cut the fillet at the end of the piece (only at one of the four corners). This all fits with your explanation.
I can live with the defect in the part. I need to cut more of the parts so I may raise the stock higher so the Z axis has less flex.
Thanks.

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