Found it. Second last post on this thread, with potentially the same issue you are having. f4w2hdudoc software to edit the FITS header and remove any bayer matrix info incorrectly written there.
With APP it is the other way around, you have to remember to tick the debayer box each time if you are using OSC data or it stacks as mono.
https://www.iceinspace.com.au/forum/...=DSS+mono+RGGB