XamTabControl doesn't call ContentTemplateSelector

Well, in my particular case I was using a Composite Collection, which your example does not utilize. So that might be a difference.
			<TabControl
				x:Name="tabs"
				Grid.Row="1"
				TabStripPlacement="Left"
				HorizontalContentAlignment="Stretch"
				VerticalContentAlignment="Stretch"
				Padding="0"
				>
 
				<TabControl.Resources>
					<DataTemplate x:Key="ServiceStepTemplate">
						<local:ServiceStepEditView ServiceStep="{Binding}"/>
					</DataTemplate>
				</TabControl.Resources>
 
				<TabControl.ItemTemplate>
					<DataTemplate>
						<Border CornerRadius="10">
							<Grid HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
								<TextBlock
									Text="{Binding Description}"
									Foreground="{Binding Status, Converter={local:StepStatusToBrushConverter}}"
									x:Name="description"
									HorizontalAlignment="Right" VerticalAlignment="Center"/>
							</Grid>
						</Border>
					</DataTemplate>
				</TabControl.ItemTemplate>
 
				<TabControl.ContentTemplateSelector>
					<local:ServiceStepTemplateSelector/>
				</TabControl.ContentTemplateSelector>
 
				<TabControl.ItemsSource>
					<CompositeCollection>
						
						<TabItem Header="Call Info">
							
							<StackPanel
								Orientation="Horizontal"
								Grid.IsSharedSizeScope="True">
								<StackPanel Margin="10">
									<corewpf:BOEditor Property="{Binding ProtocolCode}" ComboBoxList="{Binding Source={StaticResource protocolListProvider}}" />
									<Grid>
										<Grid.ColumnDefinitions>
											<ColumnDefinition SharedSizeGroup="Label" />
											<ColumnDefinition SharedSizeGroup="Field" />
										</Grid.ColumnDefinitions>
										<Label Grid.Column="0" Name="label"
											Target="{Binding ElementName=patientPicker}"
										>_Patient</Label>
										<local:PatientPicker x:Name="patientPicker" Grid.Column="1" PatientId="{Binding PatientId, Mode=TwoWay}" />
									</Grid>
								
									<corewpf:BOEditor Property="{Binding CallDateTime}" />
									<corewpf:BOEditor Property="{Binding OriginalCoordinatorCode}" ComboBoxList="{Binding Source={StaticResource coordinatorListProvider}}" />
									<corewpf:BOEditor Property="{Binding CurrentCoordinatorCode}" ComboBoxList="{Binding Source={StaticResource coordinatorListProvider}}" />
									<corewpf:BOEditor Property="{Binding SourceCode}" ComboBoxList="{Binding Source={StaticResource sourceListProvider}}"/>
									<corewpf:BOEditor Property="{Binding ReferringFacilityCode}" ComboBoxList="{Binding Source={StaticResource facilityListProvider}}" />
									<corewpf:BOEditor Property="{Binding ReceivingFacilityCode}" ComboBoxList="{Binding Source={StaticResource facilityListProvider}}"/>
									<corewpf:BOEditor Property="{Binding CallerName}" />
									<corewpf:BOEditor Property="{Binding ContactTypeCode}" ComboBoxList="{Binding Source={StaticResource contactTypeListProvider}}"/>
									<corewpf:BOEditor Property="{Binding ContactName}" />
									<corewpf:BOEditor Property="{Binding PhoneTypeCode}" ComboBoxList="{Binding Source={StaticResource phoneTypeListProvider}}"/>
									<corewpf:BOEditor Property="{Binding Phone}" />
									<corewpf:BOEditor Property="{Binding PhoneExtension}" TextMinWidth="50" />
									<StackPanel Orientation="Horizontal">
										<Button 
											x:Name="CancelCallButton" 
											Click="CancelCallClick" 
											HorizontalAlignment="Left"
											IsEnabled="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=csla:ObjectStatus, AncestorLevel=1},
																	Path=IsNew, Converter={corewpf:BooleanNotConverter}}"
												>
											<AccessText>_Cancel Call</AccessText>
										</Button>
										<Button 
											x:Name="ViewStatusChangesButton" 
											Click="ViewStatusChanges" 
											HorizontalAlignment="Left"
											IsEnabled="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=csla:ObjectStatus, AncestorLevel=1},
																	Path=IsNew, Converter={corewpf:BooleanNotConverter}}"
												>
											<Image Source="..\Icons\StatusHistory.png" Height="20" />
											<Button.ToolTip>
												<ToolTip>View Status Change History</ToolTip>
											</Button.ToolTip>
										</Button>
									</StackPanel>
								</StackPanel>
								<StackPanel Margin="10">
									<corewpf:BOEditor Label="Created By" Property="{Binding CreatedUser, Mode=OneWay}" IsReadOnly="True" />
									<corewpf:BOEditor Label="Created On" Property="{Binding CreatedDate, Mode=OneWay}" IsReadOnly="True" />
									<corewpf:BOEditor Label="Modified By" Property="{Binding ModifiedUser, Mode=OneWay}" IsReadOnly="True" />
									<corewpf:BOEditor Label="Modified On" Property="{Binding ModifiedDate, Mode=OneWay}" IsReadOnly="True" />
								</StackPanel>
							</StackPanel>
						</TabItem>
						
						<CollectionContainer
							Collection="{Binding Source={StaticResource dataProvider}, Path=Steps}" />
 
						<TabItem Header="Notes">
							<Grid>
								<corewpf:NoteEditView
									Name="notes"
									KeyboardNavigation.TabNavigation="Cycle"
									NoteEditList="{Binding Notes}"/>
							</Grid>
						</TabItem>
						
					</CompositeCollection>
				</TabControl.ItemsSource>
 
			</TabControl>