Hmm, interesting, this has brought back a memory of when I was performing a test loot run to gather data for loot drops in Back to Basics, and that was in a back to back run too.
BUT, the one time difference I made there, was that I either recalled with the loot chest open in that run, or I didn't close it before I recalled out whilst next to the chest (whilst I normally pull away completely allowing the chest to close fully before recalling). And the next run when I got back there, the chest gave nothing because on opening you could see there was another chest there, the previous runs one.
I can not remember if this previous chest was visible to my other accounts that was looting now, as I just ignored it and jotted down the above actions (not closing chest or not closing and recalling), but there may be something in relation to re-running and not closing the previous "same" chest properly before the one in the new run.
Might be nothing, but putting recorded info out in case it helps.
J1NG