Not sure what triggers the issue since it doesn't always happen but Ill try my best to explain it.
Sometimes if I spell a word incorrectly, then backspace to correct it, my phone will undo everything I backspaced as if I didn't attempt to correct it at all. Never had this problem until yesterday after updated my phone
I am having the exact same issue, and it's been since I got my new phone out if box a couple weeks ago. I have been nervous that I may have gotten a defective new device, but if someone else is having this issue I feel a little better. It is even happening while I try to type this. I read something about clearing the keyboard data. I am going to give that a try.
