It has now been reportedly confirmed that another Guardians of the Galaxy Vol. 2 character survived the first round of battle with Thanos in Avengers: Infinity War.
Outside of Infinity War and Avengers 4, Guardians of the Galaxy director James Gunn has mentioned that Guardians of the Galaxy Vol. 3 will take place after the events of Avengers 4.
So, Mantis has made it through the deadly events of Avengers: Infinity War and survived long enough to start filming on Avengers 4.
It’s not clear exactly what story line, if any, that Mantis will follow in Infinity War and Avengers 4.