Friend, I had a similar problem when trying to establish a connection between an Azure Function and a Key Vault using a private endpoint. I was trying to add a secret from the app settings, and after many attempts, I came across this surprise: https://learn.microsoft.com/en-us/azure/azure-functions/flex-consumption-plan#considerations

"Key Vault references in app settings don't work when Key Vault is network access restricted, even if the function app has Virtual Network integration. The current workaround is to directly reference the Key Vault in code and read the required secrets."