FatFs strange behavior when writing to text file
I found It,

Initially I set the buffers as uint16_t because I thought to fill them with integer numbers and then I forgot it and I decided to fill them with characters G and H.

Ok, you could say that characters can be interpreted as decimal values too but, I did not cast  to integer values because I forgot they were of type integer.

So I changed the buffers from uint16_t to char type and now all data are recorded as initially expected. I don't know what happens behind the scene but I assume that it has something to do with memory addresses.. Anyway thank you all for your time.