The internal API queries made by ProofreadPage::onInfoAction() and ApiQueryProofread::execute() are hitting the action API "too-many-pageids-for-query" deprecation warning.
For most users, a maximum of 50 IDs may be specified for the pageids parameter. IDs in excess of 50 are ignored with a warning, although someday I'd like to make that an error. Ideally, instead of making internal API queries you'd use PHP code to more directly access the information you require. For ProofreadPage::onInfoAction(), expose your own operation as some business logic class instead of going through ApiMain to get to ApiQueryProofread. For ApiQueryProofread, a well-crafted SQL query would probably be more straightforward.
But if you insist, when you have more than 50 IDs to query you'll need to query them in batches.
: Users with the apihighlimits right have a limit of 500 rather than 50.