LODS only generate in terrain ive passed through

i tried using distant horizons on this aternos server, like this smp ive had for a while, like all this terrain here (including unloaded chunks) ive probably already flown through it with elytra in the past, but it only loads the one that i have passed through. also, it has this weird visual glitch, like this weird stuttering or something like particles of like color similar but slighely different to the lods that the particles appear infront of