netherrack and magma block in main world

I first created this world in snapshot 20w17a and travelled to this place, and found this unexpected structure. It even has 2 stone brick stairs blocks on the top! And I re-created this world in 20w18a, and tp to the same location to see if this happens again...again.

world seed: -8649121689888007729

coordinates are shown on the screenshot.