Reformed Egyptian is described only inside the Book of Mormon's own text, attributed to Nephite scribes modifying Egyptian characters over centuries. No inscription, manuscript or artifact outside that text has been identified as an example of it, and mainstream Egyptology and historical linguistics have found no independent evidence such a script existed, so there is nothing external against which to locate an origin.
What would resolve this An inscription or manuscript independently datable and decipherable, found outside the Book of Mormon tradition, that could be shown on linguistic or archaeological grounds to be the same writing system, would give scholars an actual artifact to locate and analyze.
OpenEgyptology and historical linguisticsJoseph Smith (translator, by his own account), The Book of Mormon