Why does my humidifier not come on when heat is on?
I found the problem while getting the wiring diagram. I'm posting my answer in case someone else comes across a similar problem.
Why it didn't work right
The humidifier was wired into the EAC terminal. This terminal only becomes active over a certain RPM and so the humidifier was only coming on when activating the fan manually, or when the heat hits stage 2. During testing the heat didn't hit stage two, so I didn't realize this.
Where it should be connected
I solved it by wiring the humidifier through the appropriate humidifier terminals on the circuit board. They were located immediately next to the EAC, and they looked the same, but the HVAC installation instructions pointed out that (in contrast to the EAC terminals) these are &quot;dry contacts&quot;. So it didn't work just switching over switching the single wire that was in the EAC terminal.
&quot;Dry contacts&quot;
I was unfamiliar with &quot;dry contacts&quot; and online descriptions were confusing to me. As far as I understand it now, it basically means it's not a hot/neutral pair of terminals. It helped me to simply think of it as an on/off switch operated by the circuit board, which is why I had to split one of the wires to the humidifier and insert the terminals in series.
My installation manual's wiring diagram

Now it's working the way I want to: the humidistat gets voltage whenever the heat and the fan are on.