As mentioned by the accepted answer, App Script functions don't produce any logs when they're evaluated as formulas. You can run the function from the App Script console, but that doesn't work if the problem is based on how it's being used in the sheet. In my case, I didn't realize that even one-column ranges are provided as 2D arrays.

Paying it forward -- the solution I found to test this was throwing Exceptions and treating them like console.log()s:

Your exception message will get printed into the tooltip, which lets you get some info out of the failed execution. You essentially get one log per run. It accepts pretty long failure messages, too, so you could try to do something clever with adding more log messages during the execution until you get to your problem area.

Wish there was a better answer, but this helped me figure out my problem!