I am loving the use of the term “unfurling”. We refer to oneboxing here for that functionality. I’ve added the onebox tag and also alerted the staff experience team to look into how the UI might be improved to make it easier to understan what’s going on in a case like this.
I believe there are other cases when the onebox displays an error message in the composer if it won’t be able to create a proper onebox. Maybe that could be a solution here as well. Or as you say just display it as a simple link if it’s not going to onebox correctly.
Looks like in your example you did not leave a line break before the URL to be oneboxed. Here it is looking as expected.