Nether portal structure spawned in the overworld

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It's not a bug, it's a feature listed in 20w16a changelog
 

RUINED PORTALS

Shattered remains of ancient nether portals. Wonder who built them?​

• They can be found in any overworld or nether biome

• Some are hidden underground, under the sea, or buried in sand

its a feature. please delete this

See MC-178986.