Is this intentional?
Yes it’s intentional. The point of a custom arena is to create artboards in any layout you want. You can set up your own design-mode-like layout by adding the same pages/components with different variants. Then move them around to your liking!
ya; I’m loving it for all of that 
I have just been designing an app that uses Collapse a lot; so not being able to use hotkeys to get into Design Mode and use the Interactive mode to uncollapse the sections takes me out of the Custom Areana