I think you misunderstood this idea behind that change. Let me explain:
AotJ gave ownership to the person casting it first.
A Healfirebrand casts AotJ and a cFB casts his AotJ 1 second later, now all allies have 5 stacks of AotJ that they will apply over the next 5 seconds (due to the icd). All of this damage would go to the healbrand. The healbrand just stole the cFBs AotJ.
This was not really an issue before the May 11th patch because AotJ had no icd. AotJ would get applied nearly instantly and it was very difficult to "steal".
After the May 11th patch they introduced a 1 second icd on AotJ stacks.
Lot's of people complained about AotJ needing to count for the individual caster. The June 8th patch was supposed to fix exactly this but nothing changed in game.
So could you explain why suddenly AotJ would behave this way without any note in the current patchnotes?
Not really. So you can assume it's a bug until the devs confirm it was an intentional change and not a bug.