The bug was first brought to my attention about four days after completing the Grid Infrastructure upgrade. The client system administrator (SA) noticed that the disk holding the Oracle home directories was slowly filling, at the rate of about 1Gb per day. We identifed that core dump files being created under the new GRID_HOME/log/
The good ol' OTN forums led me to bug 10283819. The original poster there shared also that removing the old (220.127.116.11) grid home directory and restarting diskmon services stopped the core dump creation. The poster then went to question a second issue with increased diskmon.log writing. After a solution was found for that, Oracle Support closed the bug for some reason, without ever addressing the core dump creation.
I can verify that removing the old 18.104.22.168 grid home (I did a tar+bz2 first) and restarting the services did stop the core dump creation, and am pushing back to Oracle support to get the bug re-opened or a new bug filed to specifically address this. In the meantime, if you are unable or unsure about removing the old grid infrastructure home, it should be safe to have a regularly scheduled script remove the diskmon core dump directories and save you a full disk surprise late some night.