From what I have tested on my Xbox when I go to re-edit one of the following items: command blocks, chat messages, chat commands, or renaming items in anvils, all the text you typed after the spot you edited gets deleted.

Note: from what I have tested book and quills seem to be unaffected by this bug.
Note: I did see a post about the same bug earlier, however that post seemed a bit unclear. So I reposted the same bug with a bit more information and clarity.