Spacetime around the Wellspring Sea was in a state of constant flux. It had taken even the Blazesun Ruler a full three years of travel via his warship before he had managed to pass through it. Ji Ning was at a higher level of insight than him, but it had still taken Ning a full five months before passing through the region of dangerous, distorted spacetime.
If you made a mistake when passing through a region of distorted spacetime, you could end up within a spacetime fragment and be forever unable to escape! If you were 'lucky', you might simply be teleported to an incredibly distant place… which might end up being a death trap.
This was why the Blazesun Ruler had felt so certain that he was the only person to discover the Blazesun Wellspring Sea. At first, he didn't even consider the possibility that Ning would be there. It was only after his forces had searched the entire Blazesun Domain to no avail that he had thought of this place and came here to test us luck.