This is an optical illusion. Although those two big "triangles" look like triangles, they are not actually triangles. The slopes of the hypotenuse of those two small triangles are 2/5=0.4 and 3/8=0.375, which are not equal.

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Specifically, it is

The ratio of two right angles of the red triangle is 3: 8, and that of the green triangle is 2: 5. The ratio of two right angles of these two big "triangles" is 5: 13, which is not similar to any of the above triangles, and these two triangles are all included in this big "triangle", which shows that these two big "triangles" are not triangles at all.

In fact, my previous answer shows that the slopes of these two small triangles are different (which can be understood as the inclination of the hypotenuse), resulting in the hypotenuse of the big triangle not forming a straight line. After careful observation, we can find that the hypotenuse of the above figure is concave, and the following figure is convex, which just forms a parallelogram with the hypotenuse of two small triangles as adjacent sides, and the area of this parallelogram is exactly equal to the area of the "lost" square.