Got a response from Adobe team after a ticket was opened for this issue:
The warning should be ignored
The synchronous ajax call is there by design. There are some plans to fix this by loading the file via a synchronous script tag include instead. That would both avoid the error and optimize performance. However, that enhancement won't be included in AEM until a future version such as AEM 6.5 as it has already been decided that it won't be included in AEM 6.4 (next year).
The warning was not causing any issue for us but we wanted to confirm if it was appearing because of any customization we made.