Disabling one screen, the other also disabled at the same time

[ravingdrifter]

On 2017 iMac 5K (Radeon Pro 580, macOS 10.13.2), disabling one screen the other will be also disabled at the same time. Been tested with an Acer monitor and a Samsung TV. Unplugging the HDMI to bring back iMac's screen.

[ravingdrifter]

On 2012 iMac (GeForce GTX 660M) also running macOS 10.13.2, DisableMonitor works pretty well.

I also want to mention here that my 2017 iMac hooks up external monitors via Thunderbolt 3 port with Apple's Thunderbolt 3 / USB-C to HDMI adapter, which is different than my 2012 iMac that is via Thunderbolt 1 with a Thunderbolt to DVI adapter. I believe developer knows that HDMI is a lot more difficult to deal with than DVI.