To directly answer the question,
BackdropTemplate uses its own
OnLoad handler to set the backdrop. When you supply your own, the default behavior is to overwrite it and the original no longer runs. You can call
self:OnBackdropLoaded() manually to run it in your own handler since the template still writes its function there. Alternatively, you can use the
inherit property to run your handler before or after the template's by setting it to
"prepend" or
"append" respectively.
https://wowpedia.fandom.com/wiki/XML/Scripts