It seems like there is a problem with linked blocks that contain objects that are not supposed to cast/receive shadows (unchecked from their Properties panel).
Once a file with such objects is inserted as Linked Block into a new file, the objects randomly cast/receive shadows, depending on camera angle, selection state etc.
They only behave normally if the file they are inserted into is empty, as soon as other objects and shadows are there, the non-casters start to flicker and cast shadows where not supposed to.
Here is a test file - a few boxes with cast/receive shadows OFF.
cast-receive-linked_test.3dm (1.9 MB)
To reproduce, make a new file with some random boxes and in rendered mode make sure shadows from sun and/or Skylight are enabled. Then insert > link the above file and try rotating the view, selecting/deselecting, changing sun position etc. - the linked block shadows should randomly show or dissapear. Can anyone else reproduce this problem?