Steps to reproduce:

1. Open the attached project (rect_mask_test.zip)
2. Open 'Test' scene
3. Notice it has a red image created as a child of white image with a 'Rect Mask 2D' component. Also notice that the red image is scaled to -0.5 on Y axis.
4. Move the red image down until a part of it goes out of the rect mask bounds
5. Notice the red image becomes invisible (Same happens with buttons, input fields, etc)

Note: The image only disappears if it goes out of the rect mask on the same axis it has an inverted scale (that is, if Y scale is inverted, you have to move the red image along Y axis to see it disappear, if X scale is inverted - you have to move it along X axis).