the only times SQ staff would ask me to have my seat up right is: Take off, meal service and landing..................

Sounds like you encountered a more blunt FA that sounded direct.

To be honest, swearing back at them doesnt help the situation either. Regardless of SQ, UA, US, CO, AC etc etc..... swearing back at them is just a reason for them justify your aggression / potential hazard to the safety and well being on board the flight.

I have seen people swore at the FA in UA F before take off and they were swiftly taken off the flight by a supervisor and police.

A better approach is probably having a discreet word with the inflight supervisor afterwards or to write in.