It is good security practice to NOT re-use a stream key (ie don't use a Persistent Key). But that may not be practical for some use cases
As I don't use a Persistent Key, I've not had an issue with the stream key not appearing on that page ... but Facebook has been making changes (and had a number of glitches along the way, especially this year) with the Live/Producer interface. And I've seen the FB Live/Producer page be different depending on the browser used (from a Win10 PC).