Output of AC op amp circuit has unknown superimposed signal on top of expected output signal
If you'd plot the frequency reponse, all will become clear:



The transfer function has infinite gain at 1.59k, which happens to be 318*5. I'll leave it up to you to find the transfer function (it should be in the text book). Minor hint: reduce the gain and see what happens, say make it 39k instead of 40k. This also happens because the default ideal opamp in LTspice has no output limitation, it's just a VCCS+(R||C) inside.