That is a Flash problem that occurs every now and then - no idea why.
Essentially, Flash has a function that tells you how tall the box must be, to fit all the text in. Sometimes it returns a wrong value.
I "work around" this by adding a few pixels myself, but sometimes not even that is enough. It never seems to happen to me when I test - try meddling around with quality settings, and see if switching to a different quality fixes this?
