Please refer to the attached project that reproduces the issue. In the MainPage constructor, you can switch between using the PickerEditor (validation works) and the SegmentedEditor (validation doesn't work).
The error message is not showing when the editor is empty and the ValidateAll method is called
Add TextAttributes option to DataFormEditorStyle and DataFormGroupHeaderStyle (e.g. Bold).
Add FontFamily option to DataFormEditorStyle and DataFormGroupHeaderStyle.
Expose an IsEnabled property for editors that can be toggled after DataForm is rendered. Currently, only Data Annotations can determine whether or not a property's editor is read-only. If a specific editor's read only state needs to be changed dynamically (e.g. due to a selection in another editor), you would need to completely reload the DataForm.
Available in minor release 2018.2.727.250. It will also be available in the R3 2018 release.
The expected behavior when updating the text on a button click, for example, is to expand the editor. Currently the editor is expanded when the user starts typing inside it.
Add public-facing methods to re-call DataSourceProvider GetSourceFor methods. This would allow for refreshing the datasource. Alternatively, add support for ObservableCollection to be used for DataSources so that the pickers reflect any changes to the underlying data source.