I found this issue with changing TextField.text on a TextField.type = "dynamic" with embedded fonts. Basically the text was rendered the first time correctly from the actionscript with a TextFormat applied to the TextField with an embedded font setup correctly. When attempting to rewrite the TextField.text property upon an event or the like, it appeard that the text just disappeared - similar to when the non-device font is not embedded properly.
The solution is to use this rouge, but documented, function called TextField.setNewTextFormat(). When setting the newly created TextFormat object to a TextField object of type dynamic or input, I use something like the following: