Why did my adjacent landscape glitching out at some part?

So I’m trying to make a 500km2 landscape using world composition, and when I’m importing a heightmap at a certain adjacent landscape (in PNG format, I made it in photoshop), part of the landscape is sort of glitching out. The glitched part is unsculptable in any ways.
I have made a brand new project file with Third Person template, but the same glitch occurs again.

Is this problem considered a bug? I’ve repeatedly got the same problem, even without touching advanced settings. Or do I miss something?