Bug 13976 reported that the sort by popularity was only alphabetic and not numeric. I cannot confirm that... because now it's broken. When sorting by popularity the result list is sorted by biblionumber instead, which is the usual fallback when values are identical. When searching issues:74 or popluarity:74 records where the items have this number of issues are found correctly. So it appears the index is there and filled, but somehow not working when used for sorting? Verified in 18.11 and 20.11.
See bug 13976. I also had a struggle to get it working. You need a preference or the cron job to get the sum of issues into the biblioitems.totalissues column first. Closing this report.