Since I only have 8 analog inputs on my interface (Focusrite 18i20), and several of those have external preamps on them, I like to set the inputs on tracks I've completed to "None" so I can reuse the inputs on new tracks. I don't have any problem doing that, but Sonar Platinum doesn't seem to save that info. When I save, exit and reload a project, all the the track inputs are set back to the interface. I've become used to taking a few minutes to clear out all the inputs, but it's kind of a pain and I'd rather not have to do it. Am I doing something wrong, or is this normal behavior?

From a workflow perspective, as long as the track is not armed or echoed, an "active" input will not affect the track. You may be overthinking the situation, since if you decide you want to record another take, you would then need to re-assign the input rather than simply arming the track and checking it. I think this may be intended, since when overdubbing (common for most DAW users) I will only use one input most times, but it will end up feeding multiple tracks over time, so my initial selection of input 1 - right (for mono audio in) I do want to retain so I know that arming is that input (unless I change it).

Audio track Input assignments should stay 'None' so long as the tracks are not armed and Input Echo is not enabled.