Submitting forms using the new Smart Forms features works fine for me. (It's an awesome feature to get forms working ASAP).
But I need an industrial strength form handler so I'll roll my own backend form handler (new project). However (this when the fun starts) in a bss form, if I de-select smart forms, set method to "Post", leave target and encoding default and set action to point to a suitable php file e.g. https://www.w3schools.com/action_page.php I get nada. Problem. I found a suspicious form attribute data-bss-recipient ... which seems to have no business here at this time. If I eradicate it problem solved. There seems to be a bug here, yes?