I think there is bug in the back button related to the side menu. When navigating to a state without an abstract at the root, the back button does not show. It would make sense for the button to appear given that you have not changed nav-views. The starter template for side menus demonstrates the issue, perfectly, any of the side menus items seem to clear the history and prevent the back button from appearing. Reason I noticed this was my app has a couple abstract states that are linked to children in the side menu. When going to those states, the back button works as I would expect it, but when navigating to states without a parent, no back button.
Not a bug, this is done on purpose.
Determines whether the side menu is enabled when the back button is showing. When set to false, any menuToggle will be hidden, and the user cannot swipe to open the menu. When going back to the root page of the side menu (the page without a back button visible), then any menuToggle buttons will show again, and menus will be enabled again.
This follows ui patterns seen in native mobile apps. But this configurable so it can be turned on or off.