Verification builds:
1.9.0.5 Beta | Android: Yes
1.8.1 | Android: Yes
1.8.0 | Android: Yes

Affected Devices
OnePlus One A0001 on Android 6.0.1 (CM13)
Android Moto e5 cruise/play

Summary:
When adjusting certain settings in the options menu, such as the Button Size slider, they sometimes set back to zero when pressing the back button.

Steps to Reproduce:

1. Launch Minecraft
2. Select Settings→Controls→Touch
3. Adjust the Button Size slider (Possibly helps to drag off the slider at the same time?)
4. Press the back < ui button (top left)

Observed Results:
The slider will be reset to zero (minimum) when pressing the back < ui button.

Expected Results:
The setting should not be adjusted by tapping elsewhere on the screen.

Screenshots/Videos attached: Yes

Notes:

Workaround for the issue: Use the devices back button or gesture rather than the Minecraft UI button - this enables the user to exit without accidentally adjusting the settings.