Compare https://github.com/wikimedia/mediawiki/blob/1.26.0/includes/libs/objectcache/BagOStuff.php and https://doc.wikimedia.org/mediawiki-core/1.26.0/php/classBagOStuff.html.
Method makeKey() was introduced in 1.27, yet it is listed on doc.wikimedia.org on 1.26.0 as well as other <1.27 releases. It looks like the build was triggered for tags, but isn't checking out the correct code. It seems to be falling back to a newer branch (or master even?).