Err-returning function losing wrapped value

[matteojug]

I couldn't exactly identify what is the issue, as the following test (with the commented edits) show different failures:

• as is: the compiled version seems to lose the inner value of the err:

compiled:    Ok(Some(Tuple(TupleData { type_signature: TupleTypeSignature { "fn": (response UnknownType (list 1 (string-utf8 0))), "mymap": (optional UnknownType),}, data_map: {ClarityName("fn"): Response(ResponseData { committed: false, data: Sequence(List(ListData { data: [Sequence(String(u""))], type_signature: ListTypeData { max_len: 1, entry_type: SequenceType(StringType(UTF8(StringUTF8Length(0)))) } })) }), ClarityName("mymap"): Optional(OptionalData { data: None })} })))
interpreted: Ok(Some(Tuple(TupleData { type_signature: TupleTypeSignature { "fn": (response UnknownType (list 1 (string-utf8 3))), "mymap": (optional UnknownType),}, data_map: {ClarityName("fn"): Response(ResponseData { committed: false, data: Sequence(List(ListData { data: [Sequence(String(u"foo"))], type_signature: ListTypeData { max_len: 1, entry_type: SequenceType(StringType(UTF8(StringUTF8Length(3)))) } })) }), ClarityName("mymap"): Optional(OptionalData { data: None })} })))

• uncommenting the map-set line: compiled: Err(Unchecked(InvalidCharactersDetected)) (interpreted one stays correct)
• uncommenting the map-set line + changing map-get to fetch key 1 ((map-get? mymap 1)): test passes

#[test]
fn something_funny() {
    crosscheck_compare_only(
        r#"
        (define-map mymap int int)
        (define-private (somefn)
            (begin 
                ;; (map-set mymap 0 99)
                (err (list u"foo"))
            )
        )
        { fn: (somefn), mymap: (map-get? mymap 0) }
        "#
    );
}