Hey There:
I just experienced this problem, and found no answer online. Then I discovered that intermittently, if any part of your premiere window occupies the second monitor, even a hair, you won’t be able to see the second monitor in your playback settings. It will work if this setting is already activated, but won’t work if you are trying to turn it back on. And again, intermittently a problem, though this is always how I solve it now. Bizzarre, but hope this helps!
-Matt