Not sure if this is really a bug or not, but entities such as elder guardians in ocean monuments render before the chunks containing the structure render, causing the structure to be revealed.
Linked issues
Comments 3
Please could you provide some steps on how to go about reproducing this issue? Additionally, attaching some screenshots or videos showing this would be quite useful and greatly appreciated. Thanks 🙂
This should be forward-resolved to MC-242331.
Brevort
This is actually because entities load faster than chunks, which cannot be fixed in any way.