It's not a bug, it's a feature listed in 20w16a changelog
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
I've done some more tests.1.14 is last unaffected version, bug occurs in 1.14.1 Pre-Release 1
Cyryl Waśkiewicz
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