Two queries on the same table seems to share set of fields

Two queries:

1. $ab1 = Doctrine_Query::create()
->from('Model_UserAddressbook ab')
->leftJoin('ab.UserPersons up')
->leftJoin('ab.AdminUsers au'); // second one doesn't have this join

2. $ab2 = Doctrine_Query::create()
->from('Model_UserAddressbook ab')
->leftJoin('ab.UserPersons up');

When executing the following fetchOne:

A.) $x1 = $ab2->fetchOne()->toArray(); // has fields from addressbook, and subarray UserPersons, OK
B.) $x2 = $ab1->fetchOne()->toArray(); // has fields from addressbook, and subarray UserPersons and AdminUsers, OK
C.) $x3 = $ab2->fetchOne()->toArray(); // has fields from addressbook, and subarray UserPersons and AdminUsers, NOT OK

$x1 and $x3 should have the same set of fields, but $x3 has somehow received set of fields from $x2.

I haven't turned on any caching, but even if the caching is on, the result sets shouldn't mix, right?

I'm talking just about field names (set of fields), not fields data.