from how i understood the explanation in the MSQ, once the elemental balance tips towards a flood that starts affecting the aether in the Source, but a calamitous event needs to happen on the Source at the same time for the boundaries between it and the shard to crack enough for all that excess aether to rejoin with it - the war of the magi, the dropping of dalamud, the theoretical event of the black rose being unleashed, etc. - so i don't think that a shard merely being in a flooded state is enough to cause a rejoining