Your car functioned as designed. The "lift/foot mode" Holds the gear for 20ish seconds. It is designed to keep you in the power band going around a corner, so as not to abrublty downshift half way through a corner and upset the balance of the car. If you think back, I bet the times it happened was always on a curve of some sort.
The car will shift in this mode - at redline. Or after the 20 or so seconds of easy driving. Or if you shift back into drive
09.1 White Hot, Sport/Premium