If I set a refresh interval on a feature layer, any selected feature becomes unselected when the layer refreshes. This appears to be a new bug in ArcGISRuntime 100.3. It doesn't happen in version 100.2.
The attached repro sample has two buttons. One to create a feature if there aren't any already on the layer, and one to select a feature. Click "Select Feature" and then click on a feature. It should show a blue highlight which will then disappear in a few seconds when the layer refreshes if it is using version 100.3. The selection remains unaffected when using version 100.2.
Note: You will have to add the ESRI.ArcGISRuntime.WPF NuGet package to the project. I removed it to reduce the size of the file.