In tundra I have found a tresure chest occupying the very same field as the green portal. Initially I could not pick up the chest's content due to having no space in my inventory. The chest did not vanish, therefore I did not make the portal work properly; the portal icon did not light up, when I have reached the portal field. To be able to use the portal I had to destroy part of my stock for the chest to vanish first. I have seen portal guardians standing on the portal field before, but not any other items there.

Due to having no such prior experience I am not sure, if it is a bug, oversight, intentional feature, or just something ordinary to get used to.

It happens from time to time, such as two chests appearing on the same tile, making the 2nd chest invisible.

The reason is due to how explore is randomized, so it's an oversight I guess.