What happens?:
API for "proofreadpagesinindex" exposes for continuation:

"continue": {
    "gprppiifrom": 101,
    "continue": "gprppiifrom||"
},

But when gprppiifrom is provided, API gives a warning:

warnings	"Unrecognized parameter: gprppiifrom."

In API sandbox gprppiicontinue is used for continuation instead.

See https://en.wikisource.org/wiki/Special:ApiSandbox#action=query&format=json&prop=info&list=&generator=proofreadpagesinindex&formatversion=2&gprppiicontinue=51&gprppiiprop=title&gprppiititle=Index%3ASandbox.djvu&gprppiilimit=50

What should have happened instead?:
API should provide the correct parameter to continue.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc:
1.38.0-wmf.16

When trying to extend support in pywikibot for "proofreadpagesinindex", this is an issue, as pywikibot builds the API query based on info contained in the API response.