I seem to have the exact same problem along with other problems with created dynamic text fields. I been working with flash for years and I never seen flash code act so wierd. I am building an App and it really seems that createTextField is buggy and unreliable.
mytext.embedFonts = false; text shows up fine
mytext.embedFonts = true; text dissapears yet sometimes works, how i have yet to figure that out.
From what I can tell, its a run-time error in flash. As like you said, you have to space the code through a couple frames. This makes it a nightmare for parent actionscript calls or functions.