This is not entirely unexpected as the longer fiber lengths leads to more loss and noise. However, the sharp drop off of the BER from 0 to 1 suggests that at one point the OFDM demodulator is completely wrong at detecting the input. This suggests to me that the delay of the optical signal is large enough (from the length), so that the demodulator is misinterpreting the symbols of the constellation diagram, because of this phase shift. This sensitivity is a problem with the OFDM demodulator and one the OptiSystem team is fixing in the next update coming later this year.
You might be able to work through this by changing the delay compensation parameter in the OFDM Demodulator manually.
Regards