I've never seen this before. Just installed SONAR on my new PC. The highlighted track doesn't automatically turn on the input monitor button. When I click on them they are either on or off. How do I get input monitor back to auto. There must be a setting somewhere I assume, but I can't find it. This is making SONAR very difficult to use. 

I'm also using an Apollo for the first time. Could this be disabling SONAR's auto input monitor facility?
Just asking. No idea if that is a thing.

First thing to check would be:
Hit "P"
Enable Advanced at the bottom
Under Midi (on the left) choose Playback and Recording
At the bottom check "Always Echo Current Midi Track"

Thank you Steve. Can't believe I missed this. Don't know why that had gotten turned off. Much appreciated. 

You're welcome!