If I remember correctly March 18 was the date on which the order of travel restrictions came in to being and it provided as under
Prohibition
2 Any foreign national is prohibited from entering Canada if they arrive from a foreign country other than the United States.
This does not apply to
t) a person whose application for permanent residence was approved under the Immigration and Refugee Protection Act, and who received written notice of the approval before noon, Eastern Daylight Time on March 18, 2020, but who has not yet become a permanent resident under that Act;
I also think that now this classification of applicants on the basis of date on which the order of travel restriction was passed has become redundant. All people who have unexpired visas should be allowed to fly as the people with expired visas will ask for authorisation anyway.
Canada has extended the travel ban for the month of September.