That seems familiar to me, I had some plants with those signals in the leaves and it could be due to an excessive use of fertilizers. I'm not an expert though, so maybe it's better to see more opinions about it. :).
Cheers.
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Thanks for the tip, but they were planted in regular potting soil, without anything extra added.
I have been asking around, showing the pics, and the best guess up until now is that it is leaf spot disease. Not very good news, because there is not a whole lot that can be done about it 🙄