I'm not sure this is the actual failure mode, but a quick inspection seems to point to that as an explanation.
What is the failing use case?
Not closing since this is reported by a WebKitten. But setting low priority since this is not clear what the bug is.
I think I may have confused myself on the correct behavior here. I thought event.DataTransfer.types was supposed to include "text" if "text/plain" was in the list of available types. However, upon reading the standard again, I don't know if this is the case (it may have been an older draft I'm remembering). I'll have to look into this more.
Ok. I close for now then. Please reopen when you have more info if it is a valid bug.