It's because you've chosen 180° movement in the Hue value, this providing the opposing color on the color wheel.
Then, you reduced the opacity to 50% which is attempting to find the value between two hues on opposite ends of the color spectrum. The median property being gray between two hues that are canceling each other out because they are on opposite ends of the spectrum.
If you move that opacity up even just a few percent in either direction you'll see it turn to the brown/sienna color or the blue color of your source.
So no technical glitch here, just some good old fashioned digital color theory.