The forced libtiff writer cannot preserve it either?

I don't know why the saved image is broken in main. However,

https://www.awaresystems.be/imaging/tiff/tifftags/planarconfiguration.html

The specification adds a warning that PlanarConfiguration=2 is not in widespread use and that Baseline TIFF readers are not required to support it.

So it seems like a good idea to avoid the "separate" setting for the sake of compatibility of our created images.

Yeah, it's ok to play it safe for this case. I was just wondering if there was a remote chance of preserving the original image configuration, but unfortunately don't have the time to experiment right now...