I debugged this one for hours, thinking it had something to do with my django formset code / dynamic handling of new entries.
It turns out the reason was casued by mismatching tag elements.
I two forms, one in a left column, and another larger full page form starting with the right column; thus I had a form element starting inside of a div block which wouldn’t be closed until the end of the page.
This caused the problem where the browser refused to POST the data.
This type of tag mismatch works if it’s rendered by the server, but not if you dynamically add the elements.