To cite Abstract, "Finally, in our conclusions, it is argued how the spin-flip contribution might be closely related to the occurrence of nonphysical spin operators." https://doi.org/10.3390/sym16010097

Because the light speed cannot be exceeded, such spin-flip contribution points to non-zero rest mass. So, there cannot be massless fermions. The only massless thing so far is the photon. But it is a boson.
Yes, the Standard Model talks about massless neutrinos, but experiments have shown that it is theoretically incomplete.

If spin-flip means the change of a scalar product (S=spin*momentum), then this is a pseudoscalar. It is a paradox of the theory, which points to the impossibility of zero mass fermions. In such a case, there are no spin-flips for the massive fermions (please check it). In conclusion, because a spaceship cannot move at light speed, there cannot be massless fermions. Why? Because there are fermions (physical matter) inside the ship. The only massless thing so far is the photon. It is needed to be massless for the Special Relativity to be working.