Here's a reproduction of the bug:

struct OutputRange {
    void put(const(void)[] bytes) {
        import std.stdio;
        writeln(bytes);
    }
}

unittest {
    import std.range: put;
    OutputRange rng;
    put(rng, "Hello");          // Writes [72, 101, 108, 108, 111] as expected

    import std.algorithm: map;
    put(rng, "Hello".map!((dchar a)=>a)); // Writes the bytes internally representing the MapResult: [5, 0, 0, 0, 0, 0, 0, 0, 82, 175, 72, 34, 59, 86, 0, 0]
}