The FM90 and FM90_B3 model x range in the table here:
https://dust-extinction.readthedocs.io/en/latest/dust_extinction/choose_model.html
are different from what the code tells you when you use wavelengths outside the limits.
Code says 2.87 and 11.111, table says 3.13 and 11.0.
It would be good to make sure the values in the table are consistent with the values in the code.