Same thing has been happening to my incredible since MIUI was released for the DInc a few months ago. It happens when in landscape mode and the keyboard suddenly closes. When you re-open the keyboard to continue the message, you no longer see what you are typing. The only 100% way to fix was to reboot the phone. If it happens in GTalk, I can close out of it completely, then get back into it and it's fine. With SMS, I have to reboot the phone.
More and more people are starting to have this issue.
I suggest you try the Droid X or Droid 2 keyboard for now. I found that I don't get this problem as often when using those.