Found the issue - skipped updates!

There is a KB article on it here (and how to fix it): https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB0690146

I wish it was more evident and told you it was skipping updates, like it they do when promoting Update Sets.

I guess I will just have to follow the steps in that article every time I do this.