PDF Reader: can't highlight passage, but can search for words in it

I'm working with an old text and I just came upon a passage where Zotero won't let me highlight a few sentences. So I thought the OCR hadn't worked there. However, I am able to search for words in that passage, so I don't know what's wrong?

Any ideas?
I'm on Linux Mint, Zotero 6.0.

Thanks :)
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