I was stuck and getting stubborn in trying to understand a bug, until Sean gave me a couple of useful debugging advice:
If there is a similar page where it works, look very carefully at the markup, in that case a JSON object rendered to the page, and try to find the differences, like in those games on La Settimana Enigmistica back and forth, spotting the differences. Sometimes a different key, or a lacking key could be enough to break things.
Start removing/commenting out all the code to the minimum until you find a state where it’s working. Then add things back in one at a time and find what caused the feature to break. This one I knew, and forgot after being digging the rabbit hole and being stubborn. So it’s always a good reminder.