When typing a sentence or command in chat, using the arrow keys does not move the cursor visually, meaning that you can't go back and edit the text accurately, as you cannot tell where you are in the line of text.
Steps to Reproduce
Open chat
Type a few words
Use the arrow keys to move the cursor back through the text (on screen keyboard OR external keyboard)
Notice the cursor doesn't move
Type or delete letters, and observe where the changes are made (not where the cursor is)
I've had this issue happen on Pocket Edition in 0.17.0.1, but it does not seem to occur on Windows 10 Edition as of 0.16.1. It seems to be a Pocket-exclusive bug.