Based on my investigation so far it seems the problem is caused by TemplatedParent in some capacity. Specifically, BreadcrumbBarItem WinUI template uses the following for the individual buttons:

<Button x:Name="PART_ItemButton" x:DeferLoadStrategy="Lazy" ...>
	...
	<!-- This is the Button Content -->
	  <Grid AutomationProperties.AccessibilityView="Raw">
	  <ContentPresenter x:Name="PART_ItemContentPresenter"
	    AutomationProperties.AccessibilityView="Raw"
	    Content="{TemplateBinding Content}"
	    ContentTransitions="{TemplateBinding ContentTransitions}"
	    ContentTemplate="{TemplateBinding ContentTemplate}"
	    FontFamily="{TemplateBinding FontFamily}"
	    FontSize="{TemplateBinding FontSize}"
	    FontWeight="{TemplateBinding FontWeight}"
	    HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
	    VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
	    LineHeight="20" />
	  <TextBlock x:Name="PART_EllipsisTextBlock"
	    AutomationProperties.AccessibilityView="Raw"
	    FontFamily="{ThemeResource SymbolThemeFontFamily}"
	    FontSize="{TemplateBinding FontSize}"
	    Padding="3"
	    Text="&#xE712;"
	    Visibility="Collapsed"
	    VerticalAlignment="Stretch"
	    IsTextScaleFactorEnabled="False" />
	</Grid>
</Button>

Now, this worked before #15672, but after these changes the materialization presumably happens later, so during ElementStub.Materialize the Button gets created with no TemplatedParent initially and you can see that in the logs where you will see:

Applying template binding parent  on Microsoft.UI.Xaml.Controls.ContentPresenter

Note the two spaces after "parent" in the log - this is actually "null". As a result, the ContentPresenter.Content is left null and the buttons are invisible.

I am guessing the TemplatedParent of ContentPresenter needs to be evaluated correctly as BreadcrumbBarItem immediately when the ElementStub materialization happens, as TemplatedParent should remain "constant" throughout the lifecycle. However, I am not sure how to do that

To be accurate, the button (lazy element) does receive templated-parent (TP), but not at creation. Since at the time of materialization, the relevant TemplatedParentScope was already popped, so we late inject the TP. Now, that works for the lazy element itself...

However, I had not consider the fact that, lazy element can nested descendants, and they are also materialized "out of scope". So at the best case, they receive no TP, at the worse case, receive the incorrect TP...
I've modified the code around ElementStub::MaterializeInner to push a new TP scope when lazy element materializes.