I have pulled out the metadata and they both have the same structure. I have no idea what is causing this inconsistent behaviour. On my Macbook air, they were both switching earlier today – but now neither is switching. The Big Sur wallpaper switches to match the mode.
On my iMac right now, the Catalina wallpaper shows based on the current time whatever your mode. I thought Big Sur had stopped doing this, but it turns out the behaviour is just intermittent. When you switch to dark mode, even in the middle of the day, it will show you an evening/night image. Sometimes, the operating system will honour that declaration. One of those is the feature where dynamic wallpapers can declare which images are suitable for dark mode.
Sometimes it is something that looks like it was intended as a feature. Sometimes, it is just one of Apple’s many frustrating bugs* around wallpapers. These dynamic wallpapers should change with the sun – but they don’t always. One of the trickier things that Multi Monitor Wallpaper does is pull apart dynamic wallpapers so that it can build separate ones for each of your monitors.
Tldr Multi Monitor Wallpaper can give you the daytime images for your dynamic wallpaper even if you use dark mode.
