Fp2 lod chunks seem to vanish as soon as they enter vanilla render distance. Vanilla chunks take some time to appear once they are in render distance. This causes some visible holes in the terrain. I figure this is probably known.
Observed in cubic chunks worlds in voxel mode.

I think this is fixed.

not closing because i'm pretty sure this can still occur in some circumstances, in particular when using certain optifine fog settings while moving around. there are also still some bugs where there can be a 1-block gap between vanilla and fp2 terrain.