I need an easy way to reproduce the problem. The following works:
A2 = "A1 + $A$1"
A3 = "A2 + $A$1"
var wb = new XLWorkbook();
var ws = wb.AddWorksheet("Sheet");
ws.Cell("A1").SetValue(1)
.CellBelow().SetFormulaA1("A1 + $A$1")
.CopyTo("A3");A2 = "A1 + $A$1"
A3 = "A2 + $A$1"