See https://legacy.curseforge.com/wow/addons/sod_bislisttooltip#c9 for initial issue. I've fixed this by changing ShoppingTooltip to ShoppingTooltip1 and ShoppingTooltip2.

I should review all available tooltips and attach to the relevant ones. For example, should I attach to ItemRefShoppingTooltip1 and ItemRefShoppingTooltip2?

ItemRefShoppingTooltip1 and ItemRefShoppingTooltip2 seem to be the tooltips that display when you compare items with the ones you already have equipped (they are two because of rings and trinkets).
I tried already to include those two into your tooltip hook and it worked. Much better as you can determine if your equipped item is already a higher rank from the BiS list.