Player is supposed to be in the nether while loading from the overworld to the nether.

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オーバーワールドからネザーへの読み込み中にネザーにプレイヤーが居ることになっている

ということ

I think he’s referring to that when traveling through the nether gate; there is a delay for the dimension to load and the player is able to get attacked by mobs while it loads. Think there is already a report for this issue.