Hi Sam
My first response in any situation like this is always to check that the students are using a compatible browser - Which browsers does Canvas support?
But it could also be due to the size of the student's browser window. I did a quick experiment and was able to reproduce something similar (but maybe not exactly the same) as what your students have reported. With my browser window maximised to fill the screen, my clock remained visible at the side of the page when I scrolled up and down. This can be seen in my first screenshot and is the same as @kona demonstrated. Then I resized my browser window, gradually making it smaller. Once it gets below a certain width and there is no longer room for the timer at the side, the timer moves to the very bottom of the screen - as shown in my second screenshot. In this case, the timer won't be visible when you scroll up to view the questions at the top of the screen. So it's also worth advising your students to maximise the browser window to its full size.
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