Thats why there is a simulator
No simulator is perfect, to try an issue such as this with a simulator is in effect to say "I will see if the programmer of the simulator researched this and made the simulator behave as a real chip under these conditions. In all honesty, if I were to write a '51 simulator (fortunately I am not), I most likely would have made the SBUF load a function of mov SBUF,.. not any SBUF write.

Erik