Upper Chionthar River area disabled after starting SoD

[maurvir]

In my current EET build, the upper Chionthar river area BG0400 (the farm with the zombies) was disabled (greyed out) after starting the SoD part of the game and never reenabled, even after starting the SoA content. I'm not entirely clear why it was disabled anyway, as it is far from the city of Baldur's Gate. Is this a bug, or intended?

This wasn't much of a problem in the current play through, as the party had already dealt with it, but my next run will include NToSC, and it involves a return to that area.