I created a superflat world with the preset "The void" (seed is 4712119467792282436), and next to the platform generates a "default" world.

That default world generation seems not to be from 1.13, because there is no algae or underwater plants in the ocean. (I think is 1.2 version or higher because there is some little jungle generation).

Also when you go to a biome of the default world generation, it appears that is the biome in which you are in the "F3" tab.

Years ago, it happened to me something similar in 1.5.2 but with normal superflat world. Thanks.