TextField setTextFormat() method

When I have a line of text in a TextField with portions of the text having different fonts, it seems to be breaking the text where the font changes. So, if I have “Hello World” and Hello is Font A and World is Font B, it is rendering the text like so:

Hello
World

Is this normal behavior and, if so, can I stop it? Note that I am using TextFormats to do this, not htmlText.

Thanks.