A TextFormat bug?

I wonder if this is a bug.
This is the scenario: you have a TextField to which you have applied a TextFormat. Then you try to add some text to the TextField. There are two possible outcomes of doing that: if it is an HTML TextField, the formatting is preserved after inserting the additional text, as you would expect. But if it is a non-HTML TextField, all formatting is lost when the text is added.
An example of this:
After pressing "Add Text", the String "is a long" loses its formatting in the non-HTML TextField
This is the code:

t = "this is a long text bla bla bla bla bla";

tfield1.html = true;
tfield1.border = true;
tfield1.htmlText = t;

tfield2.html = false;
tfield2.border = true;
tfield2.text = t;

var tformat = new TextFormat();
tformat.bold = true;
tformat.italic = true;

tfield1.setTextFormat( 4 ,15, tformat );
tfield2.setTextFormat( 4 ,15, tformat );


but.onRelease = function()
{
    tfield1.htmlText += " more text ";
    tfield2.htmlText += " more text ";
}