Thermo Pride OL5-85 oil furnace and Aprilaire 60 humidistat

I have a Thermopride OL5-85 furnace. I've recently added an Aprilaire 600 humidifier, which comes with the Aprilaire 60 humidistat. Wiring diagrams for both are attached below.

The humidistat is supposed to run when the furnace is running, and when the humidity is below the designated target (I am using it in Manual mode). It is not running, even in Test / Reset mode.

After trying numerous things I have traced the problem to the fact that there is no voltage between the W and C terminals, even when the furnace is running. Here's what I get when the furnace is OFF:

• Voltage between W and C (furnace is OFF): 1-2V – as expected
• Voltage between W and Rh (furnace is OFF): ~26V – as expected
• Voltage between Rh and C (furnace is OFF): ~26V – as expected

As soon as I connect W and Rh, which I've done both by turning up the temp on the thermostat, and by manually connecting the two terminals with a length of wire, the furnace turns ON, and I get the following readings:

• Voltage between W and C (furnace is ON): 1-2V – NOT as expected
• Voltage between W and Rh (furnace is ON): ~0V – as expected (after all, these are bridged directly)
• Voltage between Rh and C (furnace is ON): 1-2V – NOT as expected

Furnace seems to be running fine in every regard. I am completely stumped, because this is not only running counter to my understanding of HVAC wiring, it is running counter to my understanding of basic electricity. Help me, O DIY StackExchange, you are my last hope!

Furnace wiring:

Humidistat wiring