Ember Data - Safari doesn't finish a PATCH request on certain requests

I am using Ember 2.7.0 and Ember Data 2.7.0 to make a PATCH on a model.

I have a problem whereby with some requests in Safari, a PATCH request never completes. The debug tools show the request is still waiting for a response.

Everything works fine in Chrome/IE.

Is this a bug in Ember Data and/or Safari?