I have observed similar befuddlement by US controllers, becoming greater the further south I went with my Canadian-registered aircraft. A workaround which helped was to deliver the call sign carefully, with a modified first letter: “Canadian Golf”, pause, “Alpha Bravo Charlie”. Many of the controllers could at least reply with the “Alpha Bravo Charlie” part. And of course the controllers near Canada (KBLI, Seattle Approach, KGTF) had no trouble digesting all the letters.