That apparent arithmetic rounding, instead of using a fractional factor may be the reason for the error, later I will be testing it, on the sodaq board, after editing that library and recompile.
It is very interesting what you explain to me where the libraries are located, I did not know how to get to that sourcecode.
What was the intended objective for that conditional adjustment for speeds below 600bps?
By the way, the constants SAMPLE_RATE_ARITH_x16 and SAMPLE_RATE_FRACT_x16 where they are defined? To better understand how it works.
I clarify that my knowledge about library management and creation is very basic, especially if we talk about the Arduino core.
I will update the results for the change suggested, later in my lab.
Thanks in advance.