On mail1 app: Settings → Mailboxes (“Mailboxes settings” page):
The option Add a prefix to spam messages subject doesn’t work. The prefix is not saved (you need to re-access the section or refresh the page to see it has not been applied on UI).
If this field is related to the one with the same name on filter section, then maybe they enter in conflict (but rather seems more of an UI problem than a variable assignment problem).