@ben This is very easy to replicate on my end.
I just downloaded your North Eleven project where you can see this problem. If you look at the setup for the mobile menu you will see that the bottom is set to AUTO. With this setting if you preview in the browser, go into responsive test mode, and resize down to a size with a height less than the height of the mmobile you will see that it does not scroll.
If you then change the bottom value to ZERO and try again you will see that it now scrolls.