That's a shame. Was the cable orange? That's the supplied cable from Fostex and is one of the worst cables I've ever used.Thanks for the info.
FWIW, my friend who owns the LG V20 and I went to Guitar Center last night. They had a set of the Fostex 50 mk3 which we knew so we wanted to try them. They did not initiate high impedance device, just normal. He tried rebooting the V20 and toggling the high fi quad on and off but still a no go. The music came in way too soft and uninspiring.
Strange cause the 50s have the same ohms as the 40s which are serving you well.
If you ever go back, I'd bring a 3.5mm cable (male on both ends) and try that out.