On create event handler is called after default values are set but before any changes to attribute values are applied such as from a create object action or a web service. This is well documented here - https://docs.mendix.com/refguide/event-handlers
On a wild guess I'd assume the web service call creates the object and then changes the respective attributes to the received values. From my experience the After create event is too reliant on the order of the actions performed.
I would suggest using a Before commit event if possible, or custom logic on your web service call to ensure the "create object" action contains all the data.
Hope this helps.
This explains a lot, though logically seen it does not seem logical ...
Thanks for the quick replies, guys!