Hi Danny,
The problem that you are experiencing is because you set the header to sticky mode on mobile. You can see the subtle difference.
If you are at the very top of the page that the sticky mode is not kicked in yet you will see that it is possible to scroll the menu items inside the menu Off-Canvas.
But if you scroll down and then open up the menu, now the header is in sticky mode and it is not possible to scroll the menus within.
Unfortunately, there is no way that you can enable the scrolling for the menu when it is in sticky mode.
I suggest that you consider changing your design, either do not use the sticky mode for the mobile or try to use an element such as Navigation Layered and add some of your menu items inside the other one as a child.
I am also curious if the Navigation modal element will work in Sticky mode as I did not test that. That is another idea that you can test.
Thank you for your understanding.