Key 7 is bugged. The past update, keys 1-10 were given only one box each, EXCEPT key 7. There are (apparently) two boxes to loot at the end of key 7. If you loot the wrong chest first, you will only get half of your loot. So far I've lost ~650 ped of loot running two key 7s because I didn't see a second chest (this was actual loot lost, in addition to the regular amount you lose due to TT returns). Do not loot the big chest upstairs first. If you do, you will be kicked out of the instance before getting the chance to loot the second chest. I sent a support case but I don't know if MA will help since they're not obligated to reimburse cash you lose due to bugs. However, they do sometimes reimburse you if the looting attempt was somehow bugged, so I hope that's what they'll do in this case.