Power Fx (gotta use the cool keyword!) JSON function in Power Apps is pure magic, it’s like a compactor converting anything you throw into it into a neat text representation to send over the wire, engrave on a golden bullion, or stash it as an alphabet soup. It works great apart from the times when it does not.
Turns out, it’s got a 0.5MB size limit. Give it anything that produces longer string than that and get blank in return. No error, no messages, no trace. Nothing. Blank. Blankinness. Nothingness. Ouch.
Now you know.