Admittedly this will be a hard thing to come across in plain TFC, but vanilla's terrain generation (and basically everything else about vanilla) works completely fine at distances like these in modern versions.

After 16,777,216 blocks out, this becomes very, very obvious:

Fixable by either switching out the affected 32-bit floats with 64-bit doubles, or switching from a float to an int or long if the decimal part is irrelevant and only the integer part is actually used.

"somewhere"

Is this... is this Terrafirma's version of the Far Lands? x3

Not fixing in 1.18 as it's a pain in the ass, would require re-doing the work completely, and would constitute binary breaking literally everything.