wpf textbox errortemplates in a Managed Addin Framework Addin

wpf textbox errortemplates in a Managed Addin Framework Addin

I've created a textbox style to contain a validation.errortemplate for use
in a UserControl class. If the UserControl is loaded in the normal,
non-MAF way, I can see the validation rule kicking in and getting visual
feedback (thick red border, a circular bang to the right of the textbox,
and an error message-in-a-tooltip) -- everything works the way I expect it
to.
However... if I load that same UserControl in as an AddIn, I lose the
visuals. (I do see the error tooltip behaving correctly, so I know my
validation rule is firing; I just don't see the border and bang symbol.
My AddIn, by the way, is based on MSDN's example for an
'addin-which-provides-a-usercontrol.'
I know a few of the limitations of an AddIn (e.g., video won't play in an
AddIn UserControl); is this another limitation, or am I screwing up?
Thanks in advance!