I’ve never heard of that problem before, but I don’t think the higher voltage in itself would cause an oscillation. But to answer your question, there is not an easy way to reduce the voltage in the Cortado. Your can try to tinker with the zobel network (R7 and C3) to see if you can nullify the oscillation, but even then, I doubt that’s going to have any affect on it because I don’t think it’s coming from instability in the XLR cable. My thoughts are that you are somehow hearing a lower harmonic from the switch mode power supply of your converter. SMPS’s can be pretty finicky if they don’t get the proper load they want to see…in which case they can go into “burst mode” and become extra noisy….and Cortado’s do present a very low load. Plug in another condenser mic to a different channel on the converter at the same time to give the 48V rail a bit more of a load. The Mackie has a linear power supply, so it’s much more stable.
Good luck.
-Brach