[sticklord]

I'm not entirely sure, especially when it comes to metatables, but it seems like it stores the functions to the table DEFAULT_OBJECTIVE_TRACKER_MODULE and then creates a link to these with the metatable, so it still links to the old function in the memory if you dont hook it before the metatable is created. And it seems SetHeader is used only once per trackerframe on startup so I believe you cant hook it (theres a good chance Im wrong here though).

Maybe you can do something like this though

local trackers = {
	QUEST_TRACKER_MODULE,
	SCENARIO_CONTENT_TRACKER_MODULE,
	SCENARIO_TRACKER_MODULE,
}

for i,t in pairs(trackers) do
	t.Text:SetJustifyH('CENTER')
end

That is if it's the header you want to edit.