OK, first off, let me say that I had this running perfectly with another Xero account with absolutely no issues.
I am using the PayrollAU API with PHP.
I have since created a new Xero account.
My process is;

I am using Xero Invoice API POST request with PHP, specifically the updateOrCreateInvoices function in the Xero API to sync transactions. However, I’ve noticed that everytime I run this function, some contact details are being updated or overwritten, particularly the “additional persons” field. This field gets cleared out after the sync process.

I’ve been working on integrating Xero API for syncing up transactional records to and from an ERP but I keep facing an error when attempting to sync transactions from the ERP to Xero which response with a 500.
I’ve narrowed down the issue to the function getTaxRates is called, specifically AccountingObjectSerializer::deserialize and I am really confused as to why this was the case.