While everyone seems to be talking about pi/4, it seems pretty clear from the graphs that OP meant pi/2.

And the issue is that this is a discontinuity. So while the red graph "correctly" demonstrates that the function's value at pi/2 is zero, it does so by showing a whole bunch of false values: the sharp vertical lines are simply incorrect. (The vertical lines in the black graph are similarly incorrect)

You can't see this in the first graph because whatever method you are using to choose x values is not choosing a value close enough to pi/2 to return 0.