The shipwreck generates with a weird mix of different types of wood and split y-level. It is separated by chunks, two neighbouring chunks can have different types of wood. (see attachments)

It happened in beach and snowy beach biome in a single-biome world. It happens frequently. 

Seems no happening in ocean biome (not confirmed)

Attachments are screenshots of 3 different glitched shipwreck

Seed: 1388443120081392988

Version: 1.16.4

World type: Single biome: beach

Coordinates: x: 12051 z: 780, x: 12480 z: 928 and the third one I didn't record it

Reproduce: 

1. Create a single-biome world with biome beach or snowy beach

2. find shipwrecks