On Android, the splash screen image is limited in size and cannot be set to any arbitrary value: https://developer.android.com/develop/ui/views/launch/splash-screen

In Android 12, if your icon is bigger than the required size, it'll be cut off in a circle.

App icon with an icon background: This should be 240×240 dp, and fit within a circle of 160 dp in diameter. App icon without an icon background: This should be 288×288 dp, and fit within a circle of 192 dp in diameter.

Therefore, the cause of this issue should be that your icon size is out of range.

Workaround:
https://github.com/dotnet/maui/issues/9794