Consider app extensions executable projects.

dotnet/targets/Xamarin.Shared.Sdk.TargetFrameworkInference.props

@@ -12,7 +12,7 @@
 
   <!-- Set the default TargetPlatformVersion if it wasn't specified in the TargetFramework variable. This must be set after importing Sdk.targets (in Xamarin.Shared.Sdk.targets) -->
   <PropertyGroup>
-    <TargetPlatformVersion Condition="'$(TargetPlatformVersion)' == '' And '$(OutputType)' == 'Exe'">$(_XamarinTargetPlatformVersionExecutable)</TargetPlatformVersion>
-    <TargetPlatformVersion Condition="'$(TargetPlatformVersion)' == '' And '$(OutputType)' != 'Exe'">$(_XamarinTargetPlatformVersionLibrary)</TargetPlatformVersion>
+    <TargetPlatformVersion Condition="'$(TargetPlatformVersion)' == '' And ('$(OutputType)' == 'Exe' Or '$(IsAppExtension)' == 'true')">$(_XamarinTargetPlatformVersionExecutable)</TargetPlatformVersion>
+    <TargetPlatformVersion Condition="'$(TargetPlatformVersion)' == ''">$(_XamarinTargetPlatformVersionLibrary)</TargetPlatformVersion>
   </PropertyGroup>
 </Project>