... due to Parsoid/Extension_API#No_support_for_global_ordering?
Topic on Extension talk:NumerAlpha
I almost can't imagine MediaWiki wouldn't have a solution for something so fundamental. It may be worth having a look at the Cite extension, which is another extension that needs to assign numbers sequentially, but has been rewritten to make it Parsoid-proof in the future.
Edit: I went ahead and created a task: https://phabricator.wikimedia.org/T337319