Thanks for any help with this everyone! 
OnMouseEnter() straight up does nothing for some reason when I have this camera in the scene. I have my regular camera which moves around, and I have a 2nd orthogonal camera for something else (rendering particle systems).
Already confirmed that the raycast is hitting the right objects, and also that there are no other colliders that are being hit. I can enable / disable this cam during playmode and all my scripts that use OnMouseEnter will stop / start working.
Any clues why having this 2nd cam prevents OnMouseEnter()?