So, a few weeks ago I was looking up some sources on the death of Miriam in preparation for a D'var Torah, and came across a very brief commentary that she died "to teach that death is mysterious" (or words to that effect). Unfortunately I did not make a note of where I found it, and now I am unable to re-discover it. It does not seem to be at Why did Miriam die in the desert? or What do we learn from Miriam's demise being juxtaposed to the Red Heifer?, although the former does link to a torah.org commentary that makes essentially the same point -- but without any sources. I am 99% sure I encountered it in one of the meforshim, but I can't find it in Rashi or Ramban, which pretty much exhausts my textual skills. Can anybody help me track it down?