One possible explanation for what you observed, could be that your function is not derivable two times. It looks as if there are jumps in the 1st derivative around the extrema. If so, the 2nd derivative of the function doesn't really exist and the plot you get higly depends on how the library handles such places.

Consider the following picture of a non-smooth function, that jumps from 0.5 to -0.5 for all x in {1, 2, ....}. It's slope is 1 in all places except when x is an integer. If you'd try to plot it's derivative, you would probably see a straight line at y=1, which can be easily misinterpreted because if someone just looks at this plot, they could think the function is completely linear and starts from -infinity to +infinity.

If your results are produced by a neural net which uses RELU, you can try to do the same with the sigmoid activation function. I suppose you won't see that many spikes with this function.