I have also noticed a bug where if I load a game and my army hasn't moved for that turn, I can usually only move 1 or 2 spaces no matter the terrain or the amount of movement points I have. The armies receive their normal move bonuses the next turn though.
There is also one terrain (I think it may be swamp) that looks like plains that slows you down.
My advice is to find an area you know has plains and see how far your guy can move back and forth until he runs out of movement points. That should hopefully let you know if it is a bug or the terrain.