I have an unlit material using world position offset that casts no shadow when it should cast one.
I have shadows with unlit without WPO, lit with WPO, but not with unlit+WPO.
Unlit shading model disables shadowcasting.
Unlit materials by definition do not interact with light, so they don’t cast shadows. However you can usually get what you want by using a lit material, setting BaseColor and Specular to 0, then just keep your EmissiveColor input.