On Android, hitting the "Back" key while in a Chest GUI (the size of the chest doesn't matter) will not only close the Inventory screen, but also throw you back to the title screen. This is not the same as quitting from the pause menu, as the current world will run in the background.
Linked issues
is duplicated by 5
MCPE-4884
Chest Crash
Resolved
MCPE-4923
Back Button bug with chests [Dupe&Solved]
Resolved
MCPE-5079
Multiple worlds at same time(using chest, maybe android only)
Resolved
MCPE-5102
On android when pressing back button in chest closes to main menu and creates player movement bug.
Resolved
MCPE-5139
Duplicate from MCPE-4878
Resolved
fixed!