Swimming on Water makes some of Blocks to not Appear Correctly.
This is the bug I found that When I'm Swimming and Toggled Third-Person View.
In the Picture you can see the Blocks Disappearing.
Also, I think no Seeds and Coordinates are Required.
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However, this issue is a Duplicate of MC-63070.
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They are "disappearing" because they are in unloaded chunks. Nothing to with swimming or 3rd person view at all.