I'm trying to just do what I did in 1.16 with fog:

@SubscribeEvent(priority = EventPriority.NORMAL, receiveCanceled = true)
    public static void onFogDensityEvent(EntityViewRenderEvent.FogDensity event) {
        event.setDensity(densityFloat);
        event.setCanceled(true);
    }

    @SubscribeEvent(priority = EventPriority.NORMAL, receiveCanceled = true)
    public static void onFogColorsEvent(EntityViewRenderEvent.FogColors event) {
            event.setRed(0);
            event.setGreen(0);
            event.setBlue(0);
    }

with the 

densityFloat

variable that changes, and it worked perfectly.

Now I did the same (copy-paste) and with densityFloat set to 0 I have a dense fog anyway, changing the value of densityFloat the fog remains the same

if you set the density to 0, there is no Fog

It should be so, but look:

may density to 90 you need to test a bit around

Seems like the more the density variable is, the less fog density really is in game

then this is not create via a fog

In 1.16 it was... this doesn't make sense