The camera preview is rendered by the GPU directly to the HDMI/DSI/DPI/composite video output, not via the Linux framebuffer.
Assuming you are using any of the SPI linked touchscreens, the the GPU has no knowledge of those.
Look for fbcp for cloning fb0 to fb1 -
https://github.com/notro/fbtft/wiki/Fra ... -mirroring seems to be some reasonable instructions.
Software Engineer at Raspberry Pi Trading. Views expressed are still personal views.
I'm not interested in doing contracts for bespoke functionality - please don't ask.