The bug

Macro entries, which start with $ in an mcfunction file, cannot include $( as a literal because they are attempted to be parsed as parameters, not as literal segments.

Workaround

You can workaround this by instantiating a macro with the argument $(.

$say $(a)

When you run the following command,

function macro {a: "$("}

$say $(a) will become say $( after instantiation.