I note that variables in the following message are percents not numbers of pages as it should be.
$1 validated pages, $2 only proofread pages and $3 not proofread pages
E.g. if there's 1 proofread page and 4 not proofread pages, then it reads as "20 proofread pages and 80 not proofread pages".

Should it be a separate bug?

There is a similar issue raised on the English Wikisource (see: What is this tooltip trying to say? on Scriptorium.)

I think I see what happened here. Implementing T76284 changed the semantics of the title/tooltip for table.pr_quality from absolute page counts to percentages yet continued using the language strings indexed via proofreadpage-indexquality-alt shared with Special:IndexPages. As the latter usage was not similarly changed the "easist" change (i.e. not involving a new set of translations) would be to modify this single line within ProofreadPage.body.php function prepareArticle from:

	$textualAlternative = wfMessage( 'proofreadpage-indexquality-alt', $q4, $q3, $q1 );

to:

	$textualAlternative = wfMessage( 'proofreadpage-indexquality-alt', $n4, $n3, $n1 );

Change 302062 had a related patch set uploaded (by AuFCL):
Format proofreadpage-indexquality-alt using raw page counts not percentages

https://gerrit.wikimedia.org/r/302062

@Tpt : Thanks for the review. Regarding your comment:

Would be nice also to apply the same change to SpecialProofreadPages.

the whole point of this patch is to bring <pages>-transclusion into line with Special:IndexPages—thus from the perspective of this patch alone, SpecialProofreadPages is already correct. I trust this makes sense?

Change 302062 merged by jenkins-bot:
Format proofreadpage-indexquality-alt using raw page counts not percentages

https://gerrit.wikimedia.org/r/302062

Thanks @Tpt for making this change apparently painless (now please tell me if you had to fix up too many of my mistakes!) Seems to be working correctly—as far as I can tell—on each of test2wiki, enws and frws.

I am happy to close this as Resolved. Any objections?