mantissa does not work with decimal numbers where whole part is over 16 characters long

[amir-talic]

Take for example a number like:

1234567890123456.1234567890123456 formatting it with mantissa of 16 should give me exactly the same number, but it actually gives me: 1234567890123456.200000.

If no mantissa is present, I get the correct result.

I'm guessing any combination of long numbers and mantissa can be broken, as long as the combined length is over 32.
This I have not tested.