Interesting... I didn't know about this cable. And there's little info about it except the usual marketing BS. Any chance you take some picture of the conductors so we could see the structure?
Did you measure 1.2 Ohms on each of the wires? Or just on the one carrying the heater supply?
The problem with litz wire is that it's a pita to solder. Each conductor is made of a lot of small separately insulated wires. The insulation is provided by enamel. So you need heat and aggressive flux to get rid of the enamel when you tin the conductor. If not done properly you can end up with only a few wires making proper contact, the others remaining insulated, hence the higher resistance...
Google Litz wire to learn more about its principle and construction, and resolder the offending wire properly, it should solve your issue.
Litz wire has better conductivity at high frequency (>1MHz) therefore it is loved by audiophiles, but the fact remains that for DC operation it is inferior to conventional wire. So I guess for a tube mic cable, it would be better to have Litz wire just for the audio signal, and regular copper for the rest...
Axel