The bug
Lava lakes can generate with exposed lava blocks.
Seed: 6927193748404355117
/execute in minecraft:overworld run tp @s 11043.32 -51.73 1491.06 -1665.17 21.19
/execute in minecraft:overworld run tp @s 10871.03 -50.25 1524.20 -1642.02 29.59
The cause
From @unknown in MC-216837.
The new caves, the noise caves, dont generate cave air. They generate normal air. The lava that generates under y -53, only generates in place of cave air. This means that lava can generate weirdly if normal caves intersect with the noise caves low in the world.
Can confirm.