Read-while loop with 2-field tabulated data: cannot print second variable before first

Question

I have a 2-field csv file with the following values:

subID,Task
A,BLOCK
B,BLOCK
C,Finger

Reading via a read-while loop, if I print the two fields in order...

while IFS=',' read -r -a vars ; do
    echo ${vars[0]} ${vars[1]}
done < some.csv

 A BLOCK
 B BLOCK
 C Finger

But if I reverse the print order...

while IFS=',' read -r -a vars ; do
    echo ${vars[1]} ${vars[0]}
done < some.csv

  A
  B
  C

In fact, any text before ${vars[1]} will be ignored:

while IFS=',' read -r -a vars ; do
    echo foo bar ${vars[1]} ${vars[0]} foo bar
done < some.csv

  A foo bar
  B foo bar
  C foo bar

Just what is going on? Is BLOCK some sort of control character in bash?