I suspect this is because the energy cost for moving onto a hill square is greater than for moving onto a plains square. After testing this a bit, I estimate the energy cost is around 300-400% normal for moving onto hills (so you get several chances to act before a monster who just stepped onto a hill, assuming you don't immediately step on a hill yourself). If you happen upon a wilderness encounter while flying over mountains (and it doesn't break your game) then you can see a more extreme version of this effect with mountain squares.

Yes, actually it's a feature. Some terrain types are costly to move into and thus you also need to be careful when moving into them.

But this leads to an actual improvement: The path finding code so far didn't take into account that some tiles are less favorable (due to this higher cost) than others... I implemented that improvement.

Fixed for ADOM II 0.3.0. Please send your real name to creator(at)ancientdomainsofmystery.com for it to be included in the credits. (If you were already asked to do so you can ignore this message. Please include a reference to the bug or RFE you are credited for.)