When I eat last meal before fajr prayer and fly East, my fast is shorter. The sun sets earlier. Is my fast valid? May Allah reward you with good. Ameen.
The ruling is to start the fast at the time of dawn of the place you are. You break your fast and have Iftar at the time of the sunset of the place you went to, the place you are. As a result, shortening the hours does not affect your fasting.