Observed: if you travel directly from the Siege Camp (or indeed from the North Forest, Forest of Mir or Marching Mountains) after defeating Yaga-Shura, then Amkethran is inaccessible until you've first visited the Oasis. However, if instead you first travel to Watcher's Keep, Deepstone Clanhold (Rasaad's ToB quest) or the Clearing (Neera's ToB quest), then from those areas you can access Amkethran immediately, without having to visit the Oasis first.
Expected: it must have been the design intention that you shouldn't be able to reach Amkethran without first visiting the Oasis, else there'd have been no point in making it initially inaccessible from the Siege Camp/North Forest/Forest of Mir/Marching Mountains.
Note: obviously the accessibility from Deepstone Clanhold and the Clearing is an EE issue. Possibly the accessibility from Watchers' Keep may already have been an oversight in original BG2, but I haven't checked that.
Related minor issue: the travel time from the Clearing to Amkethran (dodging the Oasis) is only 8 hours. That not only seems inconsistent with the geography of the map, but even means that you could contrive to arrive with long-lasting spells (such as Stoneskin) still active. Surely it ought to be a longer journey?