TextField.setTextFormat() doesn't read indexes in cs5.5

I have a TLFTextfield, which I've applied a TextFormat to, using a start and end index so it only applies to a range of text.

Something similar to this:

var textfield:TLFTextField = new TLFTextField();

textfield.text = "Some Text";

textfield.textColor = 0x0000FF;

addChild(textfield);

var tf:TextFormat = new TextFormat(null,null,0xFFFF00);

textfield.setTextFormat(tf,4,6);

From what I remember in Flash CS5, this works as it should. The text from index 4 to 6 changed to yellow while the rest stays blue.

This also still works perfect with a normal TextField.

But in Flash CS5.5, when applying a TextFormat to a TLFTextField, it applies it to the entire TextField, regardless of the startIndex and endIndex on the TextFormat. The whole phrase changes to yellow.

Has anyone else encountered this?