Confirmed that this is related to an IBM bug on the server version 10.1.3. It is not a Toad issue; it will depend on when IBM can address this on their end.
We have contacted IBM regarding this issue and they have created a bug id for it APAR IC96725, they did not provide a date when this will be address.
The APAR does mention one workaround that you can try on your SQL server end. But please check with your DBA before proceedings.
Below is the IBM Bug number and content of the bug:
IC96725: CASE EXPRESSION PREDICATES MIGHT BE INCORRECTLY REWRITTEN in DB2 v10.1 Fixpack 3APAR status
OPEN
Error description
A CASE expression predicate might be incorrectly rewritten. The
following conditions must be fulfilled to cause an incorrect
case expression predicate to occur:
1. The predicate is <CASE expression> IS NULL.
2. The CASE expression is in the form of
"CASE WHEN <col1> <relop1> <literal11> THEN <literal12> WHEN
<col2> <relop2> <literal21> THEN <literal22> WHEN <col3>
<relop3> <literal31> THEN <literal32> ... ELSE <colX> END".
If N is any positive integer number, then <relopN> can be any
comparison operator out of ("<", "<=", "<>", "=", ">=", ">") as
well as IS NULL and IS NOT NULL. <literalNM> can be any
constant, special register, host variable, or ...
