This is still a problem in 1.17-rc1. It also happens with lakes generated below y=62 (although above-ground lakes below that height on typical worlds are extremely rare).
In the image attachment showing 1.12 vs 1.17-rc1 the superflat surface is generated at y=54. This bug is especially problematic on survival superflat worlds because it renders certain resources virtually unobtainable.
This is still a problem in 1.17-rc1. It also happens with lakes generated below y=62 (although above-ground lakes below that height on typical worlds are extremely rare).
In the image attachment showing 1.12 vs 1.17-rc1 the superflat surface is generated at y=54. This bug is especially problematic on survival superflat worlds because it renders certain resources virtually unobtainable.