Several terrain available, be it stock or 3rd party, use tile-specific heightmap displacement files, distinguished by being black and white 32x32 or 64x64 bitmaps with _HM suffix. Mostly noticeable are several maps by Gepard and Israel update by me.
Currently with "standard" terrain data entries using heightmaps with high amount of displacement causes ugly strip of emptiness around the border of high detail terrain mesh, and the farther one not displacing it's mesh. Most noticeable example is modded Israel terrain.
To fix this, change this entry in terrain's _data.ini:
[HeightOffset]
LowDetailMesh=-1.5
WaterMesh=0.8
for example set this value to
[HeightOffset]
LowDetailMesh=-100
WaterMesh=0.8
this setting "moves" the far terrain mesh down by specified value, thus eliminating the gap caused by _HM files additional terrain movement. When the terrain area closes to viewpoint (or simply: You) the far terrain is replaced by higher resolution global heightmap, with tile-specific additional detail where specified by creator.
The above is example of course, generally the value you lower the "far, outer terrain" mesh should be similar to the highest displacement value used by _HM files on the map -refer to texturelist entries in specific terrain data. Values unit seems to be meters? feet?