So "ISO 8601" (with a space) does seem to be the official spelling.
Nevertheless, "ISO-8601" seems to me to be a better way of rendering the
identifier, as it makes it clear that a single identifier is intended.
I'm inclined to leave this uncorrected.
If a dash in the middle is totally unacceptable, my second choice would
be "ISO8601". That is less readable because it is difficult to
distinguish the "O" in "ISO" from a zero. But it still seems better
than separating ISO and 8601 into separate words.
Unless somebody can come up with a _compelling_ reason why it should
change, I think I'll just keep using my non-standard "ISO-8601" spelling.