The issue was a bit more involved than you initially suspected because the problem was that macro repetitions inside transcribers where basically unsupported. The linked PR #3912 fixes this particular issue for structs, but due to a lack of real world examples for other block statements I didn't check if they exhibit the same or similar issues.

If you have some more complex examples for macros, that would be much appreciated.

Edit: Well, didn't take long for me to find a problem with that implementation. No promises when I'll find time to work on this again, considering that I also don't have an idea how to resolve it currently

At first glance, I couldn't find any other macros that error out (that use repetitions) outside of this (partially censored) one:

#[macro_export]
macro_rules! metadata {
    (
        namespace $ns:literal;
        $(
            $name:ident: $ity:ty
        ),+$(,)?
    ) => {
        #[derive(Debug, Clone, Default, Eq, PartialEq)]
        pub struct Metadata {
            $(
                $name: Option<$ity>
            ),+
        }

        impl $crate::database::Metadata for Metadata {
            const NAMESPACE: &'static str = $ns;
        }
    }
}

The others that I use that have repetitions currently do not error, e.g.

#[macro_export]
macro_rules! header_map {
    ($($name:literal => $value:expr),* $(,)*) => {{
        let mut headers = reqwest::header::HeaderMap::with_capacity($crate::__count!($($name)*));
        $(
            headers.insert(
                reqwest::header::HeaderName::from_static($name),
                reqwest::header::HeaderValue::from_static($value),
            );
        )*

        headers
    }};
}

Yeah, it's (multiple) macro transcribers inside other blocks such as a struct definition that are a problem here (or similarly for enums). Since they can appear just about everywhere, I'd prefer a generic solution on the syntax definition side over a specific one, but it may just not be that easy to realize because of state tracking … Will need to ponder about it a bit.