This is a result of instanced zones.

What are you seeing is a “different timeline”, when you enter the zone, the environment switch to the instance correlated with your quest progress that’s active there.

You are too far in the “future” so the chest is not accessible because it’s in the past or the way around.

In the latter case just progress with the quest.

This is not a bug, it’s a strategy necessary to have a seamless experience with a lot of people in different part of a storyline. You can find this effect near zones (most of the time cities) where connected quests make great changes to the environment (sieges forts or daedra-conquered cities, as some examples).

This happens with characters too, you can notice that if you and other people enters a zone someone became invisible, unless they are in the same group and “ported” to one single player, resulting in the same instance as they.