Ruined portals generate floating on water in any biome that is not an ocean instead of being submerged. I could not find a great example of this in 21w40a, but they can generate without even touching any solid blocks in wide rivers.
Examples:
seed: -5122813796493059606
261 64 -14337
386 68 -22726
381 65 -31362
165 76 -36399
374 68 -62029
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Found a source of this not being intended:
https://www.minecraft.net/en-us/article/minecraft-snapshot-20w16a
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

Just to clarify many portals still generate underwater in ocean biomes, but the floating ones are all in rivers, beaches, or land biomes that extend into the ocean when they shouldn't.

Affects 21w41a

Duplicates MC-214795
Has a priority so relates to i guess

Affects 1.18.1

This occurs since 1.16.

Can confirm in 1.21, happen in my survival world.
Seed: 4255508838743998524
Coord: -500 63 803
Screenshot:

Confirmed in 1.21.3