Your simulate posting does not work properly for an AP journal (invoice) when journal name is setup with a line limit of 1. See here the details. It was freaking so difficult to find the solution for it.

https://community.dynamics.com/365/financeandoperations/f/dynamics-365-for-finance-and-operations-forum/361705/simulate-posting-is-actually-posting-invoice-in-d365-finance-and-operations