Not sure I understand your question - are you talking about putting the fuel switch to RUN early during a start, or suddenly putting fuel back to run while the engine is spooling down after shutdown?
Assuming you mean during start, pretty much nothing unusual will happen. While I don't think it's considered full blown 'autostart', the LEAP has a lot of start protection logic that will protect the engine if you do something stupid like turn on fuel at too low of an N2 during a ground start. Further, during start, if the engine is still warm the LEAP will go into a 'bowed rotor' mitigation where it dry motors N2 prior to fuel ON - so it won't turn on fuel regardless.