Sure it has happened. We see many posts of people arriving at the POE with the wrong covid tests and being denied entry. Not sure about the Doha flight but the airline can’t overrule the entry requirements into a country. Not sure why they would risk allowing anyone on the flight without a PCR test because they will need to absorb the cost of your return flight. The requirements may change but would stick the the stated covid testing requirements in the Canadian government website,has this happened with anyone or you are just stating information from the ircc website?