The vflex has a status LED that shows negotiation status. When it fails it will still output 5V, though, they don't seem to have any mosfet to switch the output off entirely.

https://werewolf.us/

I don't think I've ever seen one of those type of converters output anything but 5V upon negotiation failure. Which one did you use that did that? Their logic being "pick the closest available voltage" I presume?