Metaphysics is the study of reality and existence. Meta- being above, beyond, on top of, stretching across,
a good definition is:
“the study of the nature of the human mind, the definition and meaning of existence, or the nature of space, time, and/or causality”
So if you are questioning the nature of physics, or some of the processes-of physics that allow fundamentals of physics to be challenged, you’re in the realm of metaphysics.