A blue triangle is added to the corner of gradient shapes

I think the issue for the rectangle in question is that your Stop 1 position for that rectangle is at the Stop Position of 100% and the second stop is set to 0%. All the other rectangles have Stop 1 positioned at 0% and Stop 2 at 100%.

You might try something like Stop 1 at 35% and Stop 2 at 100% for that first rectangle. Either that or swap the Stop 1 and 2 colors and go with 0% and 100%, respectively, to achieve the desired effect.