I believe this issue is incorrectly marked as invalid. I can use other Android virtual keyboards and have the same issue. It appears the Minecraft UI is capturing the arrow key inputs to move between UI elements even when in a text editing context.
Combined with MCPE-174360, this means some edits are impossible because the insertion caret cannot be sufficiently controlled.
I had a situation when writing in a book where I could not find a valid location to click that would place the caret after the last character to append new text. Android issue MCPE-179036 further complicates the situation, as arrow key inputs do not appear to be captured by the text input context.