Is there any place in your application where this can be reproduced?

I've tried with a simple xaml in XamlPlayer or XamlToy and disabled items cannot be focused/selected with the arrow keys:

<Grid

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

<ListBox Width="200" Height="200" FontSize="20" KeyboardNavigation.DirectionalNavigation="Contained">

<ListBoxItem Content="Item 1"/>

<ListBoxItem Content="Item 2"/>

<ListBoxItem Content="Item 3" IsEnabled="False"/>

<ListBoxItem Content="Item 4"/>

<ListBoxItem Content="Item 5"/>

<ListBoxItem Content="Item 6" IsEnabled="False"/>

<ListBoxItem Content="Item 7"/>

<ListBoxItem Content="Item 8" IsEnabled="False"/>

<ListBoxItem Content="Item 9" IsEnabled="False"/>

</ListBox>

</Grid>

Sadly, this issue was reported by an intern that no longer works with us and I can't figure out where this might have bee happening.
I believe we worked around the issue and so would need to revert something to make it happen again.
Anyway, I think we can close this for now.