So, I did a bit of poking around.
I think this might be an inevitable and harmless issue, based on what I'm finding.
Since the MiMi is waterproof, you have air sealed in the internal cavity.
The air expands with heat - the heat generated from use. Since the air cannot escape (or at least not fast enough), this exerts more pressure on the outer skin, causing it to expand.
I am not sure if I would call this a "flaw" or not.
You could sacrifice the waterproof design and solve the problem by letting the expanding air escape. Removing internal air seems tricky, I don't know all about the mechanical aspects, but it would at least cause a pressure imbalance the opposite way - the skin getting sucked IN.
Personally, I'd almost say to nix the airtightness and allow some circulation. You could allow the buttons to act as vents in some capacity and it would still be easy to clean.
Or, some of the MiMi might run hotter than others or too hot.
Just some guesses.