I have this kind of lag chunk border on my server and it seems to go on to the world border, I also have thousands in one of my single player worlds ( one on nearly every chunk ) it's driving me insane. Hope it gets resolved soon.
it just happened to me. I played Realms yesterday and it worked, nothing can fix it... Except Mojang so please fix it Mojang.
I remember it also worked in the 1.14 snapshots
No, I forgot to say that it is a Realms server and also when it happens my character stays online until I log back in the next day. It takes two attempts though because the first time it tells me I logged in from another location, the second attempt it lets me join