It used to be Canadian visitors entered (unless otherwise told) with an automatic D/S so no they didn't get hit with an overstay. They would only have issues a immigration Judge found that they overstayed. Not so recently someone on here mentioned that changed and is no longer the case.
Montreal is not only hard with domicile, but vaccines also. Any other country a K1 does not need to get their vaccines to complete the medical but in Canada you will not complete the medical without having all the vaccines. They will not even send it to Montreal they hold onto it, until you fax them proof you got the vaccines. Most clinics for the medical also do not offer any vaccines so you must go to your own doctor or health unit/clinic to get them.
My own thoughts on why, is because there is no reason at all to not be up to date on vaccines if you're in Canada. They are free and given in school and the doctors office. (I'm not counting those who object to vaccines in this)
As for Domicile we have seen more then a few cases where they get the visa and then the couple doesn't bother to move for one reason or another, some will even live in both countries under the radar so to speak. The consulate is aware of these situations and wants to make sure they are serious and since it's so easy to make belief they are set up in the US, but truly still living in Canada.
This past year though Montreal has not been anywhere near as strict as they have been in the past. But I also noticed a few cases where the USC was not totally up front about living in Canada and were never asked about domicile. I think this was just part of them being more lax lately.