A number of USA stations, mainly operating mobile, operate from Canada without indicating the country identifier in their callsign.
These stations do not comply with the terms of RBR-4, so QSOs with these stations are mapped as INVALID by Club Log.
"9.2 The operator of an amateur station licensed by the Government of
the United States shall identify the station:
a. by transmitting the call sign assigned to the licensee's
station by the Federal Communications Commission;
b. if transmitting:
by radiotelephony, by adding the word "mobile" or "portable", or
by radiotelegraphy, by adding an oblique character ("/"); and
c. by adding the Canadian amateur call sign prefix set out in
Column I of an item of Schedule IV for the geographical location of
the station set out in Column II of that item."
Thanks to Jim, AD1C.