when a full shipwreck spawns, it will quite often have one part removed and replaced by an identical part. however, this new part will be generated as if it were newly added. normally this would not be an issue due to the fact that most parts of a shipwreck are based on what chunk they generated in, but the wood type is different every time a shipwreck is generated. as a result you get jarring mixes of dark oak and spruce. alongside this the new part is usually a block lower then the other part. i don't know why this is. on seed 2160515921293791396, world type large biomes, you can find one of these shipwrecks right next to spawn.