fbprops = FrameBufferProperties(FrameBufferProperties.get_default())
Don’t do this. Create an empty FrameBufferProperties, and only override the properties you need. FilterManager already takes care of the defaults for you.
Incidentally, FilterManager also automatically sets the appropriate aux bits in FrameBufferProperties if you pass in an aux texture.
The reason it doesn’t work is likely because the default FrameBufferProperties include a request for back buffers. Offscreen buffers don’t have or need back buffers.