Had the same kind of problem on a planet surface mission, the chest stood behind a turret in a way that there was no possible route to it, char tried to walk there and stood one pixel away from the position he would be able to open it from

after achivieng the mission objective the destroyed turret disappeared and i was able to get to the chest


this also happens from time to time without involved chests, the turrets spawn at a position where there is no possible way to get behind them, logically the one using the turret wouldn't even be able to get behind it