`format_macro_matchers` unusual behavior with commas

For a struct-like macro:

macro_rules! foo {
    (
        foo: $foo:literal,
        bar: $bar:literal,
        baz: $baz:literal,
        qux: $qux:literal,
        quux: $quux:literal,
        corge: $corge:literal,
    ) => {};
}

Formatting with format_macro_matchers = true produces weird results:

macro_rules! foo {
    (
        foo:
        $foo:literal,bar:
        $bar:literal,baz:
        $baz:literal,qux:
        $qux:literal,quux:
        $quux:literal,corge:
        $corge:literal,
    ) => {};
}

It seems like it always removes whitespace after a comma rather than using it as a possible break location. Single-line macros have similarly weird results:

macro_rules! foo {
    (foo: $foo:ident,bar: $bar:ident,baz: $baz:ident,qux: $qux:ident,) => {};
}

rustfmt 1.7.0-nightly (f704f3b9 2023-12-19)

Start by reproducing the multiline and single-line macro examples with format_macro_matchers=true, then trace the macro matcher formatting path in rustfmt. Done means commas are treated as possible break locations rather than having following whitespace removed, and both examples produce sensible formatting.