The smoking gun is this letter from Lorentz to Poincare (using automated translation, grammar not fixed):
Leiden, March 8, 1906Max Planck criticized the accuracy of Kaufmann's results, and a couple of years later, the experiments were shown to be consistent with relativity.
Mr. and honored colleague
It is already too long since I neglected to thank you for the important paper on the dynamics of the electron you kindly sent me. Needless to say I have studied with great interest and I was very happy to see my conclusions confirmed by your considerations. Unfortunately my hypothesis of the flattening of the electrons is in contradiction with the results of new experiments of Kaufmann and I think being forced to abandon it, so I'm at the end of my Latin and it seems impossible to establish a theory which requires the complete absence of an effect of translation on the electromagnetic and optical phenomena.
I would be delighted if you could clarify the difficulties that arise again.
Please accept, dear colleague, the assurance of my sincere devotion.
I think that it is to Lorentz's credit that he recognized Poincare's contribution to relativity, and accepted that an experiment could disprove it. This is good physics, not proof of an inability to accept new ideas.