I just upgraded to 22.5.0 build 45 [2016.05]... I assume this is operator error somehow, but when I try to automate volume, the automation envelope appears, but no volume changes are recorded as I move the slider. I imagine something got unchecked, or unassigned, but I'm not sure where to look. I can add nodes and make changes that way, but that's not ideal.

Hi. Sorry if this is obvious, but is read enabled checked on the track? 

All the best.

Yep

And in the Mix Module? That is all I could think of now

Mix Module?

Try this:
Click on the "W" (Write) button in track pane.
Start playback (record is not necessary).
Move the slider / volume fader.
 

https://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=ControlBar.06.html
 

Added:
Works here as expected, SONAR [2016.05] build 45.

Mix Module is new to me, but it seems to be set up as expected

I was able to do the mix I needed, but I had to go in and literally create every individual node... time consuming... I hope someone has an answer at some point.