I've come across this quite often. I do the tarir meta and when I go downstairs to loot the chests they are all already opened. I'm usually the only exalted armor at west or south and therefore basicly carry the whole meta. It is a real bummer then to see you won't get any rewards for it.
Also I know you are working on titles for metas. Is there a way to see now how many times I have done a specific meta? I Can't find anything for it on https://gw2efficiency.com/user/api-keys.