To reproduce: run dpkg-reconfigure setserial and change it to autosave once or autosave always. Run setserial /dev/ttyS0 to show the current state of your serial port (change device node as appropriate). This should usually show its UART status as 16550A. Now run setserial /dev/ttyS0 uart none and reboot.
Result: setserial /dev/ttyS0 still shows uart 16550A
Expected result: setserial /dev/ttyS0 should show uart none as it has been configured to always autosave the last configuration of the serial port and restore it on startup.
To reproduce: run dpkg-reconfigure setserial and change it to autosave once or autosave always. Run setserial /dev/ttyS0 to show the current state of your serial port (change device node as appropriate). This should usually show its UART status as 16550A. Now run setserial /dev/ttyS0 uart none and reboot.
Result: setserial /dev/ttyS0 still shows uart 16550A
Expected result: setserial /dev/ttyS0 should show uart none as it has been configured to always autosave the last configuration of the serial port and restore it on startup.