Make a bug report. I'm assuming this is mobile.
For client breaks like this, a formal bug report (including logs if at all possible, for anyone on a desktop!) are absolutely invaluable. There are so many possible variables, and every piece of information about how the user got to where they are helps the team narrow down the search.
In this case, another great thing to note is, when you restart, do you come back to the sideboarding screen or somewhere else? Have you experienced this - or NOT experienced it - in any other Bo3 environment, like Traditional Standard Play?
any chance you could shed some light on what it is about sideboarding in particular that seems to be a bit bug-prone?
not flaming, just curious.
edit: oh, and what's up with the Ogre Battledriver deck boxes?
That's definitely outside my expertise, but just speaking from How Things Tend to Work in development, the sideboarding process just involves a lot of stuff - multiple rapid scene transitions, saving deck states, preserving the board snapshot, etc etc etc. Also, best-of-three still just gets played a lot less, so it can just take longer for strange corner-case issues to surface.
The box art is even further outside my space. My best guess is, he just really wanted to be the default and nobody was brave enough to tell him no. :o