Although the forum allows names with spaces - eg, "Einar Thorsrud" - the automatic lookup when you type the '@' to mention someone does not work with spaces!
Just found another one!
User "NYP NB-IOT" has a "display" name with a space - so that can't be @ Mentioned.
But the "hidden" name is "nyp-nb_2d00_iot" - which also cannot be @ Mentioned because it has a hyphen in !!