Oh, you didn’t put spaces around "…", but your code example put spaces…
#show math.equation: set align(start)
= Without spaces
$ A union B = {x : x in A"或"x in B} $
$ A union B = {x : x in A"抑或"x in B} $
$ A union B = {x : x in A#[或]x in B} $
$ A union B = {x : x in A#[抑或]x in B} $
= With spaces
$ A union B = {x : x in A "或" x in B} $
$ A union B = {x : x in A "抑或" x in B} $
$ A union B = {x : x in A #[或] x in B} $
$ A union B = {x : x in A #[抑或] x in B} $
Do you have any evidence?