I’m not 100% sure on this because I haven’t tried such a thing myself, but try to disable writing to the alpha channel on the framebuffer (e.g. with glColorMask). The alpha channel in the buffer that is being drawn to is not necessary for drawing transparent stuff, but if you write the values there if changes how you surface opacity (I would guess).
see https://www.khronos.org/registry/OpenGL-Refpages/es2.0/xhtml/glColorMask.xml