`anyOf` generated code cannot be serialized

In our codebase, we are using the following structs, which are part of an ETH RPC API built on top of our Filecoin node implementation:

#[derive(PartialEq, Debug, Clone, Serialize, Deserialize, Default, JsonSchema)]
#[serde(rename_all = "camelCase")]
pub enum Predefined {
    Earliest,
    Pending,
    #[default]
    Latest,
    Safe,
    Finalized,
}

#[derive(PartialEq, Debug, Clone, Serialize, Deserialize, JsonSchema)]
#[serde(rename_all = "camelCase")]
pub struct BlockNumber {
    block_number: EthInt64,
}

#[derive(PartialEq, Debug, Clone, Serialize, Deserialize, JsonSchema)]
#[serde(rename_all = "camelCase")]
pub struct BlockHash {
    block_hash: EthHash,
    require_canonical: bool,
}

#[derive(PartialEq, Debug, Clone, Serialize, Deserialize, JsonSchema)]
#[serde(untagged)]
pub enum BlockNumberOrHash {
    #[schemars(with = "String")]
    PredefinedBlock(Predefined),
    BlockNumber(EthInt64),
    BlockHash(EthHash),
    BlockNumberObject(BlockNumber),
    BlockHashObject(BlockHash),
}

We're using typify to create a JSON OpenRPC specification to establish a Common Node API.

Our issue is that the untagged BlockNumberOrHash enum is translated into a definition that uses anyOf (see attached json file). The Rust code generated from this JSON specification is used in some tooling, and when trying to use the string variant to create an RPC request, we encounter the following error:

        couldn't serialize params
        can only flatten structs and maps (got a string)

Do you have any ideas on how to work around this kind of issue? For example, could we generate a oneOf (XOR) constraint instead?

Many thanks.

my_types.json

Start with the attached my_types.json and the Rust generated from its anyOf definition; reproduce serialization of the string variant in an RPC request. Trace how the generated type handles the untagged enum and flattening, then verify that the reported serialization failure is resolved or that a documented workaround is established.