ATTENTION ANDROID (especially LG) USERS:
I have a solution thanks to @I_AM_SPARTACUS
Go to settings > display > home touch buttons > hide home touch buttons > find DS & toggle it off & back on. If it's off then simply toggle it on.
And boom you have full resolution again.
I'm told you'll need to do this any time the game updates but since gree can't seem to find the problem to fix it it's a good 2nd best.
ios users this may work for you as well but I'm not sure as you don't have the "home touch buttons" that android uses.