Once "Save page" is clicked (checking changes or previewing has the same effect), the page starts to load for around a minute and then reloads, losing the changes made.
You can see an example by editing this certain page with a form on my wiki. (The page is a sandbox, but we will keep its contents as long as this task is open)
It loads for a moment but succeeds most of the time.
Can this issue be fixed? Editing the page source directly is not feasible - the wikitext editor suffers severe lag issues on long pages like these (this is honestly the reason we started using a form), so we would need a way to edit this page, and others with similar length.