Okay so here are my two cents...
Gaga is not performing a new song, but she will lend this opportunity to make an announcement at the end of the performance. I'm thinking possibly a Joanne Side B EP/ lead single/ tour extension announcement. She is performing The Cure possibly because it is one of the (bonus) tracks on the new project and she is using AMAs as the platform to kickstart the era and build hype.
However, I still believe her team planned to debut a new song. Guess they had