Followed this instrunctions:
https://www.mediawiki.org/wiki/Extension:ProofreadPage/Index_pagination_API

This is my test query:
https://pl.wikisource.org/wiki/Specjalna:%C5%9Arodowisko_testowe_API#action=query&format=json&prop=proofread&list=proofreadpagesinindex&formatversion=2&prppiititle=Indeks%3APL%20Dumas%20-%20Sylwandira.djvu

Expected to have at least some proofread properties like in the docs:

{
    "pageid": 23,
    "ns": 250,
    "title": "Page:Sandbox.djvu/1",
    "proofread": {
        "quality": 3,
        "quality_text": "Proofread"
    }
},

But I got none of proofread props:

{
	"pageoffset": 11,
	"pageid": 801322,
	"title": "Strona:PL Dumas - Sylwandira.djvu/11"
}

Raw url for the query:
https://pl.wikisource.org/w/api.php?action=query&format=json&prop=proofread&list=proofreadpagesinindex&formatversion=2&prppiititle=Indeks%3APL%20Dumas%20-%20Sylwandira.djvu

The index is mostly red (pages have initial text), partially yellow. See:
https://pl.wikisource.org/wiki/Indeks:PL_Dumas_-_Sylwandira.djvu

Simpler index same results:

• https://pl.wikisource.org/wiki/Indeks:Obraz_literatury_powszechnej_tom_I.djvu
• https://pl.wikisource.org/wiki/Specjalna:%C5%9Arodowisko_testowe_API#action=query&format=json&prop=proofread&list=proofreadpagesinindex&formatversion=2&prppiititle=Indeks%3AObraz%20literatury%20powszechnej%20tom%20I.djvu

BTW. I noticed that pageoffset is also wrong:

{"pageoffset":11,"pageid":801322,"title":"Strona:PL Dumas - Sylwandira.djvu/11"}

I assume pageoffset was supposed to be the same as mw.config.values.prpFormattedPageNumber and it is not.