The Field of View (FOV) changes to its default setting when you go underwater and resets to your preferred setting in the options when you come up from underwater. I expected FOV to stay the same underwater as it was above water. To test, make sure you change your FOV to something other than the default setting to see the change when you go underwater and back.

This bug does seem to appear when you go under lava, but it's pretty hard to see.

Confirmed on Samsung Galaxy S4 Phone Android 5.0.1 running MCPE 5.0.1 in single player survival and creative modes.