Hmm, it seems a bug related to reloading the game and difficulty settings has been found. Basically when saved data is loaded (which obviously includes savegames, but also map data for an area you have already been to), the AI's attack damage and auditory/visual preception is reset to Normal difficulty regardless of what it's supposed to be set at. If you're playing the game on Expert and have reloaded a save game rather than restarted the level, you'll mostly be playing at Normal difficulty (except for enemy hitpoints, those stay at the appropriate levels). Here's the full thread:
Oh, and SH64, this got me thinking: what if the problem has something to do with data that's being loaded from your savegame? Is that an area of the game you've been to before? If so, the only thing I can think of is that the save file for that map became corrupt and when it loads it the game crashes. If it's a new area, then I'm not sure what can be causing it, aside from an error in the map data (maybe try exchanging your copy of the game for a new one? Worth a shot). Only other workaround I can think of is to quickly press ESC as soon as the level loads to go to the menu, then try Restarting the level and seeing if that fixes it (assuming that you have enough time to get into the menu).