tldr; Multi Monitor Wallpaper can give you the daytime images for your dynamic wallpaper even if you use dark mode.
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. It then puts them back together again.
These dynamic wallpapers should change with the sun – but they don’t always.
Sometimes, it is just one of Apple’s many frustrating bugs* around wallpapers.
Sometimes it is something that looks like it was intended as a feature.
One of those is the feature where dynamic wallpapers can declare which images are suitable for dark mode. Sometimes, the operating system will honour that declaration. When you switch to dark mode, even in the middle of the day, it will show you an evening/night image.
I thought Big Sur had stopped doing this, but it turns out the behaviour is just intermittent. On my iMac right now, the Catalina wallpaper shows based on the current time whatever your mode. The Big Sur wallpaper switches to match the mode.
On my Macbook air, they were both switching earlier today – but now neither is switching.
I have no idea what is causing this inconsistent behaviour. I have pulled out the metadata and they both have the same structure.
However – I can fix it so that you can see all the images whatever your mode.
Multi Monitor Wallpaper now has a setting (enabled by default) which simply tells the operating system that every image is suitable for use at night. This is based on the fantastic work by Marcin Czachurski.
You can turn this off if you prefer. In that case, you’ll get Apple’s default and unpredictable behaviour. It might show dark images when you use dark mode. It might not!
*for example the popup display bug reported when Mojave was in beta here, or the bug I have reported where disconnecting and re-connecting screens can corrupt the wallpaper settings database, or the bug where picking a still desktop means that wallpapers set via the API are also stuck in still mode. I could go on…