Do you have AI Mesh? That was the problem for me. I had to reboot the whole AIMesh system, and before the nodes could reconnect, it gave me the option to use channels 120-128, so I could add 160 MHz on Channel 100. Then after the nodes had reconnected it no longer gave me the option and it showed "Auto", but it stayed connected at 160 MHz on channel 100. I guess a recent change to the firmware now necessitates this workaround, it's annoying. But it works. You need to reboot from the AI Mesh menu. Even if you don't have any nodes, try it.
Using 160 MHz on 36-64 is a bad idea imo because you're likely going to have interference from every single 5 GHz AP in your vicinity. I guess it depends on how densely populated your area is, but I think in most cases you won't see a speed increase vs. 80 MHz, and you're also adding 3 dB of noise, so you'll have worse reception. And moreover, the connection will be less stable, adjacent channel interference especially is really bad.
