Here is the solution (had the same issue, solution is tested and confirmed):
The displacement is relative to the horizontal vertex spacing and due to the compression in the HFZ file format, which allows a small height encoding error (+/- 1% of the horizontal scaling).
Do this in L3DT:
go to the ‘File->Format preferences’ option in the L3DT menu,
select the ‘Project maps->Heightfield->HFF (L3DT)’ tree option,
Press the ‘options’ button, and set ‘DataSize’ to 4 and ‘FloatFlag’ to true (double-click to edit these values), then press OK to clear the options window,
press the ‘Make default’ and ‘Use format’ buttons, and ‘OK’ to clear the format preferences window,
re-generate the heightmap, and;
duplicate the tile borders (under ‘operations’, ‘heightfield’…).
Using the old HFF format in floating point mode means there is no possibility of compression-related quantisation errors.
Export the heightfield in tiles in the desired format again, reimport to UE4, and you should have a 100 % precise and gap-free result.