Oracle 11g Release 1 vs. 2 — different behavior of LEFT OUTER JOIN
So I am going to answer my own question, namely: "Am I doing something wrong or is this an Oracle bug?" with This is an Oracle bug.
I leave it up to you to navigate the insanity known as support.oracle.com, but, as pointed out by @AdamHawkes, this bug is probably addressed in a recent Oracle patch set. A couple of bugs in the release notes to 11.2.0.3 seem similar to my problem, though not exactly the same.
I'll try to come back here and update the answer when I get the latest patches applied (I am in an environment where I am not in control of this).
Just linking in some related posts for other readers..
https://forums.oracle.com/forums/thread.jspa?threadID=1113096
Strange behaviour of full outer join in Oracle - how it could be explained?
Mainly suggests altering the session to turn off specific optimizations. alter session set "_optimizer_join_elimination_enabled" = false;alter session set "_optimizer_native_full_outer_join"=off;
I would rather they had automated tests that proved simple things worked and didn't just introduce these bugs in the first place.