Hmm... I am glad it works here! Yeah, as you can notice, I set the control Visible to True just before setting Focus to it, and then back to False.
By the way, just to test, I created a new StrataFrame project without any of this, dropped a RichTextbox on a form, made it bound to a BO property, run it, typed text into it and it saved just fine, without ever leaving the control. You might need to have a look if there isn't anything else going on in your code or you could write a small sample that shows this behaviour and post it here so we can have a look.