not in & NOT EXISTS... SUGAR and SUGAR CANE

NOT EXISTS can yield different results from NOT IN.
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NOT IN implies that data exists in the column. A predicate testing a
column containing a NULL value will not satisfy the NOT IN condition.
(verified on UDB LUW 8.1)

A null value in a code will satisfy the NOT EXISTS clause in your second
example. This is true even if t2 has a row containing a null value in eid!