Thank you!Sorry for the delayed response!
You are correct that Lavender x Buff would produce a black bird, likely with moderate to heavy buff leakage depending on the genes that the buff parent has. When a bird is 'split' to something, typically that's referring to a specific gene, not a full pattern. So Buff split to lavender just means that it's a Buff bird that carries the lavender gene, not that it has the genes to produce a solid Lavender coloration in its offspring. Such a bird could come from crossing Blue Cream to Buff.