It's not an exclusive thing that happens to this raid only, so it's a more in depth issue, hence why there's little progress on it.
I've done chest openings sequentially when I run 12 man Raid runs for testing, and I've seen the no item occurance once so far (in Fire on Thunder Peak), but I can no longer recollect if it was done on Reaper, I do not believe so however, but could easily be mistaken also.
But the primary takeaway is, it's not something you can just replicate by doing many times or through variations of player actions; there's something else that has fallen through that has made this possible. And without factoring that in and finding out what that is to replicate the issue concisely, there's not going to be much progress on this front.
J1NG