I have the same issue. Last night I carried two heroic treasures (lemurian statue and bountiful chest) to my base and within coffer distance.
This morning they have vanished without a trace. The treasures still appear on the treasure tally screen but the placable items themselves have vanished. Event log has nothing.
The minor treasures I found and placed are still present. Only the heroic treasures have this problem.
ps. Rather pointless to go through all that trouble to carry it to my base if its going to vanish…