It seems that when formcreate is submitted, it generates a blank page with an invisible form and an auto-submit-on-load JS script on it, as a way of generating a POST redirect. The form (which points to the normal page edit form, action=submit) does not include the CSRF token, nor the unicode check, so the form submission fails.
Surely there are better ways than this to redirect a form to another one. But the very least, the redirect form should include the CSRF and Unicode tokens.
Page Forms - 4.3 (c35e352)
MediaWiki - 1.30.0 (830bb58)