Forms that are ajaxified using the phpbb.ajaxify() function set the data-clicked attribute to true for the currently clicked submit button. This attribute is removed from other submit buttons upon triggering the click event.
However, if the submit buttons are outside of the same HTML element, e.g. different fieldsets or paragraphs, the data-clicked attribute will no longer be removed. This will cause unpredictable behavior and might corrupt any form submissions.