I admit that I’ve been annoyed with that myself. In a quick test trying to force the issue with your steps (*), this seems to behaving as expected here. Which version are you running at the moment?
(*) There’s some fuzziness between steps 3 and 4. Just to be verbose about my steps: When I press the text field button (step 3), the Text dialog box disappears and the Text Field dialog appears. I then click the Select Object button to get the area of a curve. All dialog boxes disappear and I select a curve. The Text Field dialog box reappears and I click the OK button. At that point, I’m back in the Text dialog and there’s only one string in the text box.
-wim
Go through the field process, in this case getting an area of the rectangle. Note the original (deleted text) is displayed again, and the field is concatenated onto the end.
This happens in both the Edit Text dialogue and the text Properties panel.
I just noticed another aspect to this. The deleted text reappears if focus is lost from either the Edit Text dialogue or the Properties panel. So I suspect this may be some failsafe to avoid zero value text.