ya, its a pretty short timeframe too. We had one member go AFK, so since we knew the chests were on a timer we pushed ahead without them. We cleared to it quickly, and looted it about 3mins before he came back from AFK (he went AFK in the zone as well, at the previous boss we beat). By the time he got to the chest, it wouldnt let him loot it either. It kinda stinks, but as far as we can tell, its intended.
There are two factors at work here
1) You have a limited time after every boss to get the chest. If you come back the next day or take forever to get to the next boss, the chest won't be open. Usually what we do after we kill a boss near the end of raid time is that we will stay an extra 10 minutes or so to clear to the chest since it won't be available the next day.
2) People that are not in range when you open the chest won't be able to get anything from the chest. There is no timer, if they are not in range the moment the chest is opened, they are SOL for that chest.