If the map background is bound to a property that has no initial value, the map will ignore the mouse unless using a child control. I only found one mention of anything simliar to this: https://arstechnica.com/civis/viewtopic.php?t=83073
The described behavior is expected for the WPF Framework. In this case, the initial value of the Brush property is null. When the Background is Null, the framework won't considered it for hit testing, and thus it won't be eligible for mouse events. You can observe the same behavior if you bind the Background property of a StackPanel framework element. If you don't want to set an initial color you can set the Background to Transparent. Setting it to Transparent the mouse events will be called.
I have prepared a sample project to demonstrate this behavior. When you run the project, you can observe 3 StackPanels. The first one has a Null background, the second has a Transparent background and the third one has a Blue background. You can observe that the mouse down event is not called for the first StackPanel.
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