Can you change back to when a rule is changed, it updates everything for that supplier or customer in the inbox, or at least if you change a rule when you have the next invoice if you press Apply, that change will apply to that invoice?
I have done a split rule for a customer who charges annual fees, I have uploaded all their sales invoices, only to then realise that they have increased all their prices. I now have to go into each invoice to change each line on the split. Not sure why this was changed to only new items in the inbox?