I am sorry but I had never seen such an inconsistent behaviour before! I am stunned. Look at these results::
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One single source line and the output is wriiten twice and differently!
Query contains mapping for
FIELD1: SRC.MAT
FIELD2: SRC.DESCR
FIELD3: SRC.MAT || SRC.DESCR > result is null
Query_2 contains mapping for
FIELD1: Query.FIELD1
FIELD2: Query.FIELD2
FIELD3: Query.FIELD1 || Query.FIELD2 > result is 'l1'