Trying to copy a JSONValue via .toString.parseJSON, see 
https://forum.dlang.org/thread/tsmllrkvdkuskroxcpcc@forum.dlang.org
found an error during conversion to string: 

If a value is a floating type, which ends with a .0 during conversion to string, the part after the point is cut away and the value is interpreted as an integer afterwards. 

void main()
{
    import std.json; 
    import std.stdio; 

    string s = "{\"rating\": 3.0 }";
    JSONValue j = parseJSON(s);
    
    assert(j["rating"].type == JSON_TYPE.FLOAT); 
    
    writeln(j.toString); 
    
    j = j.toString.parseJSON; 
    
    assert(j["rating"].type != JSON_TYPE.FLOAT); 
    assert(j["rating"].type == JSON_TYPE.INTEGER); 
}

If the value is like "3.1" the bug doesn't appear.