Why does the instrument assignment API fail when attempting to assign bank account to a second site?

Summary:

Hi, I have an integration that's using REST APIs to a create supplier and a bank account, and then assign the bank account to the supplier site using the instrumentAssignments API. This has been working without issue for a number of years, so I'm pretty sure I have the external payee ID for the site and external bank ID sourced correctly, and the JSON payload for instrument assignment being built correctly.

I'm now trying to use the instrumentAssignments API to assign the same bank account to a different site on the same supplier. Both sites remain active, and the instrument assignment is not end dated on the original site (but the second site will become primary pay in subsequent steps).