Looks like it's also running from the
OnShow handler on the template. You'll have to update the pointer after you hook the function. Since the hook is made to only apply to that one castbar, you only need to update the function pointer on that one.
Code:
TargetFrameSpellBar:SetScript("OnShow",Target_Spellbar_AdjustPosition);
Note: The original function only repositions the anchor. Race conditions may happen if you start putting other modifications in there.