I bought a Marumi 100,000 ND filter in anticipation of tomorrow's eclipse, and while testing the filter today I noticed the photos have an uneven but consistent purple haze, but only with the filter.
I'm using a Nikon D800 with a 70-200mm lens.
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Light is leaking into the DSLR body through the viewfinder. If you close the view finder window (using the eyepiece switch on the D800, or cover it another way..) the purple haze goes away:
Based on Why do pictures of fire come out purple?, I'd guess that the issue is specifically infra red light leakage.
While the other answer about the view finder may be correct in this situation, it's common for cheaper ND filters to leave a uneven magenta color cast.