Since change I883d9531d1, MediaWiki core provides a "vuex" ResourceLoader module. However, WikibaseLexeme also registers a module of that same name (but only in “repo” mode, i. e. on Wikidata and Test Wikidata). These modules contain incompatible versions of Vuex: Core ships v3, WikibaseLexeme v2.

Apparently, MediaWiki doesn’t report this as an error or anything, you’ll just silently get one of the versions (the WikibaseLexeme one, on my test wiki). This should be fixed before any code headed for Wikidata starts using "vuex" and expects it to produce Vuex v3.