Making a huge map of our world with a friend, it has become apparent that there are multiple problems with their rendering. Joined here you will find examples of :

• Multiple columns on the west updating long after they've actually been explored (up to multiple minutes sometimes)
• Almost half of the westernmost column not coming in even after 20 minutes of reconnecting repeatedly, turning the Realm on and off, staying around for a while and running around for a while
• Some pixels refusing to fill up
• Pixels updating in a seemingly random order
• EDIT : According to proof submitted in the comments, the pixels also don't update fast enough when you're going too fast (EG with Elytra) leaving blanks spaces everywhere

Please understand that these are not isolated issues, these are consistent across multiple maps of multiple different scales (especially the one about columns on the west edge really lagging behind) and they overall make what is a pretty easy and fun procedure in Java Edition quite a chore to go through.