2000 Sovereign with Strange Staining

Hi all, first post here!
I've recently acquired this coin from eBay, graded MS67 (Millenium First Strike) from PCGS. Coin appears legitimate looking at PCGS database, but I haven't seen staining like this on any of my other Sovereigns (see pictures).
Any idea what this is? Is it likely a fake?
If anyone has any insight I'd really appreciate it!
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Comments
Odd. I wonder if was dipped prior to submission and improperly rinsed?
Certainly looks like the coin was exposed to a contaminant. Maybe resubmit for a cleaning? Peace Roy
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