Two shipwreck structures overlap at identical coordinates (deterministic seed)

In a newly generated vanilla Java Edition 1.21.11 world, two separate shipwreck structures generate at the same coordinates, overlapping and combining into a single structure. The overlapping shipwrecks use different wood palettes and templates (one oak/birch variant and one spruce/dark oak variant), confirming that two independent shipwreck structures were placed instead of one. This is not a visual or lighting issue. The blocks are physically present, loot containers function normally, and the behavior is deterministic. Creating a new world with the same seed reproduces the overlapping shipwreck at the same coordinates every time.