Ruined portals spawn in lava lakes in the Nether. I imagine this is not on purpose as it would make them very hard and inconvenient to use as well as hard to spot. See the attachment. Note: I surrounded the portal with blocks to make it easier to see.
Attachments
Comments 3
in the changelog it says that they can be hidden in lakes https://www.minecraft.net/en-us/article/minecraft-snapshot-20w16a. So I think that this is not a bug.
Can you attach the seed and the coordinates?