There are a number of areas where drop down lists lengths are restricted with an inadequate scroll facility and no search facility so if the purchase invoice you are looking for does not fall within the list shown you cannot allocate a payment or credit to it.
This happens in Receipt hub - Hold on supplier Account and in Tagging payments to suppliers in bank accounts if you have more than the 20ish outstanding invoices it displays.
The initial list of outstanding purchases across all suppliers does indeed only show you the most recent 20 or so, but if you “pay down multiple invoices” and enter the supplier name then that per-supplier list should give you access to them all.
As always in UI design there’s a tension between making the common case easy vs making the less common cases possible. In the overwhelming majority of situations most people will only have a small number of outstanding purchases that exactly match a specific amount, so the developers have taken the decision to streamline that flow at the expense of adding one extra step to the less frequent cases.
Right click on the dropdown, choose Inspect Element, find the dropdown in the code, and adjust one of the options to be your desired length. Close the inspection code window, then click the dropdown, and select your amended option. It only lasts until you reload the page, but it’s a helpful way of getting what you want in the short term! (see orange circle in screenshot of the changes.