I've found this 9 months ago. It is a spawn which haven't got any adjacent spawns explored next to it yet. But that should be impossible, because it is not allowed to teleport (by portal) to that spawn if it was not explored before, probably by walking into it from one of its neighbours.
Can someone explain this?
Only shows up if someone entered the spawn. So the way they did it is walk around the spawn in many realms than go into that spawn which makes it visible now. It isn't by realm but by spawn that it's explored. Those "Undiscovered Realms" are the best to build bases in
ꆜᦔꫀ᧒ꪖ ꪜꪊ乂 ቿꋖቿ 乂ϟᏩჩЄϟꆜ, it would be too easy if it worked the way you said, but it does not, in fact.
You dont need to actually enter the spawn area for the realm to be shown as explored! It's enough to enter the realm's first chunk (relative coords -25 and 24) to make the realm appear as explored on the map.