[Fizzlemizz]

It has to do with the way the files are ordered in the .toc (again)

We changed it to XML first so the .lua code could "see" AMD_TabOnCharacterFrame

Now, when it's created, you're asking AMD_TabOnCharacterFrame to call a function in your .lua (which hasn't been loaded yet)

Instead of using an OnLoad script in the XML just do whatever you were going to do to AMD_TabOnCharacterFrame in the .lua.

hooksecurefunc("CharacterFrameTab_OnClick", function(self, button)
    if self:GetID() == TabID then
        ToggleCharacter("AMD_TabOnCharacterFrame")
    end
end)
--[[ 
function AMD_OnLoad(self)
    print("AddOn loaded")
end
]]--
 
-- Instead of the AMD_OnLoad function:
AMD_TabOnCharacterFrame_NextButton:SetText("OnLoad Change")

Depending on what the end goal is there might be a better alternative but this should work for now.