A Canvas.keyReleased() event is sent to the active Canvas (displayable) upon dismissal of a TextBox (lcdui). What happens here is that the box is closed upon keyPressed and the (subsequent) keyReleased is sent to the next displayable. A better solution would be to self-contain the key events within the TextBox (it is modal after all), by acting upon the keyReleased event.
As a side note, what a stunning piece of software you guys did ;)