Hi KanadaliPilot, this is an known issue since Bixby 2.0. Also there were some members already submitted this request to tech support. But for some reason still not fix yet. The easy solution is just say one more word: What is the weather today in Montreal in Celcius, then Bixby will tell you the temperature in C.
@Kwijeya open up Bixby Home and select the 3 dots on the upper right, then select quick command. From there you enter the top one as what you would say and the bottom one is the action you want triggered.
Unintentionally deleted original comment but basically create a Quick command for Weather in celsius so you no longer have to say Celsius you just have to ask whats the weather