InvokePlatformCodeDemos fails to find partial method implementations when DeviceOrientationService moved to a new Maui project

I am trying to implement partial classes for cross platform access, and although this demo works out of the box, as soon as I move DeviceOrientationService and the platform implementations into a new Maui library project, the compile complains:

/Users/dfgdfggdf/Projects/maui-samples/6.0/PlatformIntegration/InvokePlatformCodeDemos/Demo1/Services/DeviceOrientationService.cs(42,42): Error CS8795: Partial method 'DeviceOrientationService.GetOrientation()' must have an implementation part because it has accessibility modifiers. (CS8795) (Demo1)

I can't see anything different in the .csproj (other than the app config) so am at a loss as to the cause of this.

Any suggestions? Trying to implement this in a real world scenario where not all code is in a single project.

Also trying to reduce code duplication where most of our Mac and iOS code would be identical, but haven't got this basic change working yet to even try that.

@davidbritch any ideas?