It seems like {{NUMBEROFACTIVEUSERS}} (and active users on Special:Statistics) is primarily updated via updateSpecialPages.php (i.e. $wgSpecialPageCacheUpdates callback) [To be clear, this does not include Special:ActiveUsers which goes through a different process]
Wikis with $wgMiserMode = false; generally don't run that script, hence the variable gets out of date.
We should run the cache update from the job queue from time to time for those wikis (Maybe as part of the recent changes purging job)
There was a complaint on mailing list about this https://lists.wikimedia.org/pipermail/mediawiki-l/2017-October/046906.html