Then it could be under a vassal government, that is approved by Manchuria similar to the present's autonomous region. Perhaps the Dalai Lama might even be the official head of state or a spiritual guide but not with interest directed to politics.
It doesn't seem that Tibet is receiving that much interest from Manchuria as it is still a Tier 4 nation. Maybe it actually broke off again in the chaos following the Twilight War, but was gradually integrated in Manchuria? The language of Tibet is listed as Mandarin in 2320 AD, this could be an indication of how assimilated Tibetan culture has become.