That's funny, last week I did SQ ICN-SFO, then AC SFO-YYZ.
My original ( SQ issued) BP for the AC flight had AC *G notation, but AC staff in
SFO re-issued the BP when I re-checked my luggage, and it had the AC-A notation.
I brought it back to her, she did something in the computer, and re-issued it again with
the usual SE notation.