A comment from the LQT thread:

I think I've found something that might help (tested on our 1.24.1 instance).
It appears to be related to the WikiEditor extension, specifically, the preview/changes tabs that come with it. The solution that's worked for me is to disable these tabs in LocalSettings.php using

$wgDefaultUserOptions['wikieditor-preview'] = 0;

The problem here is that inputbox creates a form to submit the page, but it's nested inside the edit form on preview. Forms can not be nested in HTML. This is a problem of WikiEditor, since normal preview is displayed outside of the edit form

It uses hidden input fields to create the page, but those are interfering with the edit/preview form, so the "create" parameter of the inputbox is seen when you submit the edit form and it's in fact creating it.

I imagine that filling any arbitrary title on the inputbox will instead edit that page instead of the main page (see T64975)

The proper solution is to make WikiEditor preview feature to be injected outside of the edit form, and not inside it.

Long term this implementation will disappear most likely.

It's just not worth it to do it now however, since some non-WMF people like using it, and there isn't an alternative yet. Additionally, the whole 'editor' story of old toolbar vs. WikiEditor vs. CodeEditor vs. Mobile editor vs Visual Editor, is up in the air right now. Developers and the Editing team have this on their 'this needs serious rework, consolidation and modernization'-radar, but how and when is still very much undefined.