how ????

Because the rings of base levels aren't perfect rings. The base, if it is not a 2nd base, is just inside the 19 ring, next to the 20 ring.

nonsence the bubbles fall at level 18 bases... The whole server is another great shame of Envision,

starting to early its equal for them, level 42 bases in center and level 45 guardian bases and level 42 as highest pois. Response from envision --> nothing, if that is the surprise of a wcs world , pls programm famerama, all has nothing to do with a wcs or good playing, and again only money is importent for them, nothing more....

I've checked with the developers and you're correct it is normally at ring 18 - but it can depend on the sizing of the world.

The point at which a bubble goes is calculated from the surrounding bases and rounded down. So if it doesn't exceed 18 then the bubble will stay.

could he have relocated from another sector?

He would have had to have been even deeper into the world to end up there, unless he relocated then moved. With small bases the cooldown time is short. So yes, it is possible.

The developers have looked into it and said looks like it is because of the way the ring for losing the bubble is calculated.

there is a bug around (since a few months ago):
coordinates are decentred... some world sectors have an advantage... not the players!

he relocated so he doesn't lose the bubble for 24 hours unless he attacks a player