Based on the coordinates, that sounds like the location of the old Mushroom Island portal. Was the nether side of it at the end of a cobblestone road, in kind of a cobble house near a lava ocean? The island itself was deleted a while ago during some map regeneration, but the Nether portal to that area remained. You may have been the first person to go through it since the deletion, so it would have generated a new portal in the old spot, but there's no island there anymore, just open ocean.
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