Dielectric loss in a substrate
"If α=0 then there should be no attenuation because the factor \$e^{−αx}\$ reduces to 1. Am I right to understand that the electric field will have the same magnitude of 5 mV/m after 50 meters?."

That's correct. Loss in the dielectric is due to any nonzero conductivity. Here sigma=0, so no loss in the dielectric. In terms of loss tangent, it is zero here, so \$\alpha_d=0\$. In terms of R,L,G,C parameters, dielectric loss is due to G, however G=0 here, so no loss. E field should continue without any change in amplitude, but with only phase change.