Fwd: CSV Variables not updated

[RL]

I have a test plan like this:

Test Plan
  Thread Group
     HTTP Header Manager
        HTTP Request -1
            CSV Data Set Config -1
            -- csvDataSet1.csv
        HTTP Request - 2
            CSV Data Set Config -1
            -- csvDataSet2.csv
etc...

csvDataSet1.csv defines variables:
var1, var2, var3

csvDataSet2.csv defines variables:
var1, var4, var5

What I am seeing is that the value for var1 in csvDataSet1.csv is retained,
and not replaced by the value from csvDataSet2.csv, even though var4 and
var5 are read from the latter file.

It seems that once a value is assigned to a variable name, it is not being
overwritten with a new value until the end of the loop, even though
different CSV files are specified and contain different values for the
variable.

Is this the expected behavior, or is it a bug?
And is there a way to make it work as I was expecting and hoping it should
work: ie, that each separate CSV file would read new values into the
corresponding variables, regardless of whether they had been defined by a
previous CSV file?

Thanks for any and all help and/or suggestions!