If you try to edit an Agile board filter it will open the filter on edit mode and after you modify the JQL body you do not see the "Save" option, only the "Save As" button.

In order to be allowed to Save you need to press enter (perform a search first). As a side effect of performing the search, the "Save as" button will be transformed to "Save".

This weird behavior is counter-intuitive and I got at least 10 questions / support requests from users complaining that filter-editing-is-not-working.

In fact the cause of this is only the bad-design of the filter edit. Please fix this as it is affecting real-life usage for most users.