There seems to be a bug
I am running v.12.1.2 (build 69) on a MacBook Pro 11,5 on OSX10.11.6. When I have a second monitor set up as an extension of my desktop using the hdmi port, when I try to export a file the Export Format will not change: it's locked into whatever it was the last time. This happens both when I have that external monitor selected under the playback preference and when I have nothing selected. The export window works fine if I have mirroring selected and it works fine if I have the hdmi cable unplugged. I have tried this with a Dell monitor and a BenQ 4k monitor. Premier behaves badly if I use the hdmi port with either. The Dell is hdmi 1.3 or earlier and the BenQ is hdmi 2.0.
If I use the display port connection it works fine with the BenQ. If I use a Thunderbolt 2 connection to a BlackMagic Mini 4k Display port card in a thuderbolt 2 cardcage, it works fine as long as I have the properly updated drive. It seems to be a problem using the hdmi port at least under this operating system. I experienced something similar with the last iteration of the Program as well, but not previously.