While a bit late, this came across my feed. There is something else to consider that has not been mentioned. Many institutions will set up a unique URL for their users to sign in through. If, for example, my students do a Google search, it will take them to a generic URL to sign into Canvas for Canvas Free users. It will deny their username and password because they did not go through the student portal link set up for our district. If they try to use the Forget Password link, since they are on the wrong portal to sign in, it would not recognize their email and likely would not send them a reset link.
Something else that could be related to the issue your students are having, and that some others have mentioned having too.