The fix for JDK-8029255 removed the post barrier for setting the next field of Reference objects.
That post barrier dirtied a lot of cards that didn't really have to be dirty, but it saved us from other missed card marks. See for example JDK-8031703.
When running the Stress BPM another case of a missing barrier was detected.
The missing barrier occurs when we prune the discovered list in G1 and the list contains reference objects that are in different old regions.