I'm not exactly sure what the cap is on opening supply chests, but I wanted to suggest that it be updated to not apply to players who go through the Istan event chains normally. What I mean by that is that as long as it's possible for people to switch maps or switch characters to try to loot more chests, it likely does make sense to have a cap of some kind. But what I experienced today is I played through the Palawadan event, collected all the chests I could, then moved on to the Great Hall event which took place immediately afterward. After breaking through the gates and beating the forces inside I observed that the supply chests that appeared showed as looted already. And to be clear, I had not played the Great Hall event yet today on any character. What I suspect is that I must have reached the supply chest cap in the Palawadan event, making me ineligible to loot more chests in the Great Hall event. That wasn't a good experience, and it leaves a person wondering why the chests apparently "aren't working" or if this was a bug. If it weren't for in-game chat people telling me I must have reached the supply chest cap, I wouldn't necessarily have known why the chests weren't working.
Rather than a cap on supply chests in a time period, I'd suggest that chests simply be lootable one time only per event. That way if you switch maps or characters during that event, the chest would still be looted. But later if you participate in a different event, the chests would once again be available.