Gen 6:4 The Nephilim were on the earth in those days--and also afterward
“Gen 6:4 The Nephilim were on the earth in those days—and also afterward”
That is not necessarily a contradiction, since the Nephilim is not a term that denotes a specific lineage, but rather the type of creature. Therefore, it doesn’t say that the Nephilim who existed after the flood were in any way descended from the Nephilim who existed before the flood. Instead, they could simply have been created by the same process as the other Nephilim were created.