A fucking Django bug took more than 2h!! It was a fucking Wagtail admin site bug. If you use InlinePanel with an Orderable and use min_num=1 to have the first form open on page load, you’ll see an input named ORDER without a value in the form. Why it’s important? Because Django uses this field to order formsets and django-modelcluster depends on this ordering to make the data Orderable with sort_order field and Wagtail uses django-modelcluster. Solution? Using min_num=0. Fuck.