The mask advice didn't change because of any new evidence. It changed because the public health authorities stopped being worried about health care providers not having enough PPE. The real problem was that in the first place, instead of lying and saying masks did not work (which they knew was false at the time), they should have said "Yes, masks work, but the highest risk people are health care workers so we are using the government's eminent domain power to commandeer supplies of PPE until we know all health care workers have enough, so until that happens it will be very difficult for private individuals to obtain masks."