Kelp + Trapdoor Issue

I found kelp generating above a trapdoor in a shipwreck. The trapdoor was open so the kelp appeared to be floating. This didn't change when it was updated.

I thought that kelp only generates on solid blocks, or possibly slabs. When you try to place kelp on the trapdoors, nothing happens.

As far as I know, this occurs randomly in any shipwreck.

Seed: 56991374172103693
X: -186.529 Y: 49.18750 Z: -1138.300