When you log in many chunks dont load around you, and most will refuse to load. With some of the chunks, you an update a block inside of that chunk to render it. This bug makes serveral more smaller bugs, like forever fly, fps drops, crashing the game, etc. Sorry, even though I try and get a screenshot I can never move my mouse in minecraft due to all the lag it causes.
(Note: this seems to only happen in singleplayer worlds.)
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However, we are already tracking this issue, and this report is a Duplicate of MC-166508, which is a report that has not been resolved yet.
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