Talos that's "normal" vanilla behavior, FastCraft probably just makes it worse by loading/sending the frequently updating Chunk too fast - before the "hole" was drawn. I don't have a decent fix for this atm, but FastCraft 1.6 supports the JVM argument -Dfastcraft.distantWrsPerPass=n as a workaround, where n is the amount of chunks to update per frame for each pass. Set n to something higher than 2, which is the default, but apparently too low for your setup.
FastCraft 1.16 is available for testing at http://files.player.to/fastcraft-1.16.jar
It fixes the Optifine issue in 1.14/1.15, optimizes path finding a little and is compatible with the ColoredLights mod at some loss of performance benefits if detected.
Where I can find the download link for 1.6? Could you add that jvm argument to the newest version please? Also I cant believe how this mod improves fps a lot more than optifine with all its options, specially for people with not so good graphic cards like me.