How to reproduce:
1. Create a new project (you can switch to iOS or Android platform)
2. Open Package Manager -> set it to show preview packages
3. Install Mobile Notifications Package
4. Open Notifications settings (you can click on it in Project NotificationSettings.asset or go to Editor -> Project Settings -> Mobile Notifications)
5. Editor settings (add an icon for Android or enable iOS Push Notifications)
6. reopen the project and open Mobile Notification settings again
Result: setting are overridden to default

Tested and reproduced: 2019.1.0a10, 2019.1.0a13 (with preview.8-1.0.0, preview.5-1.0.0)
Tested and didn't reproduce: 2018.3.1f1 (with preview.8-1.0.0) <- project works just fine

NOTES:
Tested and didn't reproduce: 2019.1.0a13 (with preview.4-1.0.0 ) because it throws errors
/Users/.../PackageCache/com.unity.mobile.notifications@1.0.0-preview.4/Editor/UnityNotificationsEditorManagerEditor.cs(53,19): error CS1502: The best overloaded method match for `UnityEditor.AssetSettingsProvider.AssetSettingsProvider(string, System.Func<UnityEngine.Object>)' has some invalid arguments

and 2019.1.0a6 (preview.8-1.0.0) because it throws errors
/Users/.../PackageCache/com.unity.mobile.notifications@1.0.0-preview.8/Editor/UnityNotificationsEditorManagerEditor.cs(55,41): error CS0117: `UnityEditor.AssetSettingsProvider' does not contain a definition for `CreateProviderFromObject'