That is not the new message system. That is a problem with the message editor. It hapens when you use the keyboard shortcuts for bold/italic/etc. It seems that when you use one, it almost loses track of the cursor. Try this:
1) start typing a sentence.
2) hit CTRL+B and type some bold text.
3) hit CTRL+B again, and type some normal text.
4) use an arrow key to move the cursor forward or back.
5) see if you find where your cursor jumped to...
That's pretty much the only times if I've ever seen this behavior.
