Heat From The Bottom Vents But Cool Air At The Middle Vents ?

[ls400_1999]

Is it normal for the middle/left/right vents blowing cool air while the "leg" vents are blowing hot air? I have the control set on "auto" and the temp is set at 72 F while outside temp is around 40 F. I did not notice this problem until recently. My car ls LS400 1999 with 98K miles.

Any sugestion where I should look at? Cabin temp sensor?

Thank