I know that some people are reporting missing buttons in the visual text editor after upgrading to 2.5, but I think my problem might be different. In normal visual text edit mode, I only have one row of buttons to edit with. When I switch to full screen mode, I can see a second row of buttons for a brief moment, but then they disappear.
Sounds to me like you’re briefly seeing the buttons that are viewable after clicking the SHOW MORE BUTTONS button. If you look at your visual editor, the last button in the first row enables/disables the second row of buttons.