This bug is really hard to reproduce by the fact it is entirely random. Sometimes, weird platforms can appear around the same place. It occured to me 2 times on the same seed. One time with stone and the other time with half coral fans, half air.

Seed: 962413370393849
Command to teleport: /execute in overworld run tp @s -231.64 67.37 -185.16 -584.10 39.00

A normal generation

Stone platform 1 block above the water (y=63)

Semi-coral fan platform 1 block under the water (y=61)

Note that these platform got the size of the chunk that they are in.