'80 SC no start - 12V on pin 50 at starter switch in off?

[toddetch]

So, recently, there were a couple of times when I turned the key and nothing happened for a second or so, then the car started up. Then, I was on a drive with the local PCA club and it wouldn't start after stopping at the rest stop. Rolled it and popped the clutch to get it home.

I checked the starter, no power at pin 50 when the key is turned to start. Constant 12V at pin 30.

Here's the weird thing - at the starter switch, there is constant 12v at pin 50 until the key is turned, then it goes to 0v. That seems wrong to me, obviously. Is that a way the switch can fail? Maybe an internal short or something.

I guess I'm wondering because the switch was replaced in 2018 (I don't remember the symptoms that led to that). I came across this thread which sounds similar:
Problem with ignition switch wires

[toddetch]

I forgot to say that I do have power all the other expected places when you turn the key: radio, fuel pump, etc.