Hello, I had the same problem while reading an xlsx.
It seems that the ID is not what is mapping to the sheet. In my case the rID was providing an accurate index.
I was able to fix the problem by changing the the creation fo the xlsxworksheet:
(XlsxWorkBook.cs function ReadWorkbook)
string rid = reader.GetAttribute(A_rid);
int id = Int32.Parse(rid.Substring(3)); // the ugly substring is to remove rID from the string and keep only the number