doubt about effect of buoyancy?

Buoyancy has no effect on the weight of water or bucket.
You must remember that the loss of weight of an immersed body is not loss of weight in the strict sense. That is why the law says "apparent loss of weight", not real loss of weight!
It means that if you hold the ball with a string you will feel the loss of weight. But as far as weight of bucket is considered the ball simply adds to it.

Now, coming to your question of why the whole system does not lose weight.

In other words, what is role of the buoyancy here that causes loss of weight of ball? Well, you may consider that there is an equal and opposite reaction on the floor thus maintaining total weight.

Actually, it is due to the increased pressure caused by the increase in height of water in the bucket that is transferred to the bottom of bucket.It is interesting to note that this is true irrespective of the shape of the container.

yes, 8 kg is correct, as long as the water doesn't overflow the bucket.

buoyancy force does not reduce the weight overall. The ball displaces some of the water, and that causes the buoyancy, but the water remains in the bucket.

If nothing leaves the bucket, then the bucket and all its contents have the same mass, so it all weighs as the sum of the masses. 8 kg

The ball takes the place of as much water as it weighs. I don't know the size of the ball so I can't tell if it sinks or floats, but it doesn't matter. It will only displace AS MUCH water as it weighs or sink to the bottom if is it more dense.

The water level rises to accommodate the volume of the ball, or portion of the ball if it's floating.
No mass is lost from the system so it must weigh the sum of the parts.