Upon clicking "Replace" the mysterious response page says merely "X will be replaced with Y in Z pages" providing no way to do the actual replace. It only has a link that says "Return to form". Even more mysterious is that _some_ of Z pages are replaced but not all. Jabowery (talk) 16:47, 12 April 2021 (UTC)
Topic on Extension talk:Replace Text
It uses MediaWiki's job queue. I keep a terminal window open if I'm using Replace Text a lot and run php runJobs.php once I get to that "X will be replaced by Y" message.
Then the only "bug" is in the mysterious message which should read: "X will be replaced with Y in Z pages. See the job queue."
Someone just recently added a prominent sentence to the extension page about this.
It's also mentioned further down at Extension:Replace_Text#Things_to_know.
Yes. That's good and I did miss that but my suggested change to the message still holds -- particularly since users of the feature may not be familiar with the Mediawiki documentation and it is frequently the case that the message in question does not appear -- which can be confusing.
You could suggest it on Phabricator.
In the meantime, on your website, you could change the MediaWiki:Replacetext_success message from its default of "$1" will be replaced with "$2" in $3 pages.