Nested Loop Loses Row

$a = new CFDBFormIterator();
$b = new CFDBFormIterator();

Scenario 1:
$a->export(‘Form1’, array(‘filter’=>’Post_ID>0||Post_ID<999999’));
// Collects 2 rows
while($rowA = $a->nextRow())
{
$b->export(‘Form1’, array(‘filter’=>’Post_ID>0||Post_ID<999999’));
while($rowB = $b->nextRow())
{
(Do something…)
}
}
Result => Only the first row of $a is processed and the while loop stops at one iteration.

Scenario 2:
$a->export(‘Form1’, array());
// Collects 2 rows
while($rowA = $a->nextRow())
{
$b->export(‘Form1’, array(‘filter’=>’Post_ID>0||Post_ID<999999’));
while($rowB = $b->nextRow())
{
(Do something…)
}
}
Result => All 2 rows of $a is processed. While loop does 2 iterations because there are 2 rows in $a.

Question:
Why is the behavior between scenario 1 and scenario 2 different? The only different is the filter process in scenario 1. In scenario 1, $a actually has both rows. But the second row vanishes after $b->export line. Where as in scenario 2 nothing vanishes.

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