My organization is using Moodle 4.1.2 and I have found a CSS coding error on the grades screens on the mobile web site.
Below 768px viewport width, for some columns on grade tables the font size is reduced to 0.1rem, which makes the text unreadable on screen without doing a massive zoom in.
I have confirmed this with both Safari and Chrome on iOS as well as by using the Chrome dev tools on desktop.