By some Elsevier articles PDF, although they open correctly in Adobe Reader, opening them within Zotero shows a strange black circle covering the upper-left quartile of figures, including the Elsevier logo itself. Even more strange, this comes in black-and-white figures, not on coloured figures.

I updated Zotero, I re-linked the pdf file to Zotero, but it has not solved the problem.
Thank you for your attention.
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