I could be off-base, but my first suspicion would be the iPad used as a lightsource. I know the newer model OLED has been cited as a recommendation for scanning light source but with a specific color shift like that I’d be very curious if there are others around here, also using an iPad that do or do not see this as well.
Try a more robust lightsource. I’ve been using new Raleno unit (model PLV-S192) and have nothing bad to say about it. And it doesn’t work you can return it (though honestly I’ve been using the Raleno for many other uses, photo and otherwise) since getting it. Win win.
have a look in this long thread if you’ve not already: