There was a missing page in this DJVU file, which I was in the process of transcribing on English Wikisource. So I repaired the file (using an image from an alternate scan of the same book) and uploaded the new version at Commons. Note this is a rather large file, 978 pages and 32 MB, which may be related to the problem.
These are the two pages that are displaying incorrectly. The images that should show up should read "449" and "450" in the header of each page, respectively. But as of this writing (weeks after upload), they reflect "450" and <blank>, which are the previews from the earlier version of the Commons file.
I have purged all relevant pages at Wikisource, and tried to purge the main file at Commons, but after a minute or two I got an error "purge failed" (see attachment).
I believe the file on Commons needs to be purged in order to fix the problem with the preview in the Proofread Page view.
See relevant discussion here: https://en.wikisource.org/wiki/Wikisource:Scriptorium/Help/Archives/2019#How_to_update_the_display_of_a_DJVU_file?