With custom terrain tile.. terrain does not show, when camera is at certain position

I generated terrain tiles … and strange behavior happens…

this is normal …

but, when I move camera postion slightly…

suddenly, L:2 , X:7, Y:1 tile does not show.

and If I move camera postion slightly again…
that tile appears again…

this always happens at longitude 0 (zero).
other region is fine.

I think cesium JS will judge visibility with bounding box and Horizon Occlusion point.
so, did I miscalculate them?
or something else can cause this error?