Yes I tried walking around the map for a bit. Immediately no one was fatigued, even after going back and forth for a game time period of 2 days, after walking for a minute or two was when some of my party was fatigued, Jaheria, Khalid, and I were not, however.
This has to do with constitution. I don't know if it is really a bug or not but if you have 16+ con it is almost impossible to get fatigued. Either way it doesn't make much sense to me why anyone can stay awake and march for 3 days straight through mountains and never get tired.
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Also this topic already exists:
http://forum.baldursgate.com/discussion/11774/bug-protagonist-and-imoen-never-get-fatigue-in-my-party