Many scanned PDF books shows OCR text instead of scanned pages recently. Even those, which render properly some time ago.
Few examples:
https://commons.wikimedia.org/wiki/File:Z_pogrzebu_Mickiewicza_na_Wawelu_4go_Lipca_1890_roku.pdf :
- old thumbnall (correctly rendered): https://upload.wikimedia.org/wikipedia/commons/thumb/b/b8/Z_pogrzebu_Mickiewicza_na_Wawelu_4go_Lipca_1890_roku.pdf/page1-668px-Z_pogrzebu_Mickiewicza_na_Wawelu_4go_Lipca_1890_roku.pdf.jpg
- new thumbnall (broken rendering): https://upload.wikimedia.org/wikipedia/commons/thumb/b/b8/Z_pogrzebu_Mickiewicza_na_Wawelu_4go_Lipca_1890_roku.pdf/page1-669px-Z_pogrzebu_Mickiewicza_na_Wawelu_4go_Lipca_1890_roku.pdf.jpg
Other examplws of affected books:
https://commons.wikimedia.org/wiki/File:PL_Dumas_-_W_pa%C5%82acu_car%C3%B3w.pdf
https://commons.wikimedia.org/wiki/File:PL_Molier_-_Pocieszne_wykwintnisie.pdf
https://commons.wikimedia.org/wiki/File:David_George_Hogarth._Accidents_of_an_antiquary%27s_life._1910.pdf
https://commons.wikimedia.org/wiki/File:Eddington_A._The_Theory_of_Relativity_and_its_Influence_on_Scientific_Thought._1922.pdf
...
In the above cases old thumbnails display OK, but an attempt to generate a new one gives OCR or an empty page.