[Terenna]

It's not that you're tainting the :SetSize, it's that you're tainting SetScale()

The following code is found within MultiActionBar_Update()

local scale = 1;
    if ( contentHeight > availableSpace ) then
      scale = availableSpace / contentHeight;
    end
    MultiBarRight:SetScale(scale);
    if ( showLeft ) then
      MultiBarLeft:SetScale(scale);
    end
    VerticalMultiBarsContainer:SetSize(contentWidth * scale, contentHeight * scale)

The game tries to MultiBarRight:SetScale(), but you've set it to nil. So the game knows something is tainted. The very next function it calls is VerticalMultiBarsContainer:SetSize() which throws the taint error.

Try

MultiBarRight:hookscript('SetScale', function()
     if MultiBarRight:GetScale() ~= 0.8 then
          MultiBarRight:SetScale(0.8)
     end
end)

You might want to try and print debug that hookscript, GetScale may not return exactly 0.8, it may return like 0.799999999999 or something stupid. You might want to do

if MultiBarRight:GetScale() > 0.81 then MultiBarRight:SetScale(0.8) end