Hi I am having a weird rendering issue.

See screenshot below

I originaly thought that the sequence of the render is the issue but I re-reviewed the code countless times and it is the sequence is OK.

Background
there are 3 2d images sprite in here
1. the background image
2. the book
3. and a transparent panel

Sequence (Including RenderDoc trace)
1. the background image is rendered first to a main render target (FBO#1)

2. the the book is rendered to another FBO#2, since this is a popup window of some sort

3. then the transparent panel is rendered on top of the book image in FBO#2

4. FBO#2 is rendered as a texture to FBO#1, as seen on screen shot bellow, input is the FBO#2.

Result, the transparent panel blends with the background image instead of the book making a hole.
What could possibly wrong with it?