i usually use oiled paper for my transparencies and get that type of mark from where the paper is vs where there's nothing, or clear films.... never effects printing.... just because that part's not as opaque as the black area, but not as transparent as the clear.... so it's exposed just not as hardened as the outer parts. If it seems super under-exposed, hit it with some more light after you wash it out and it'll even it out.
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