I'm now sure that it's a bug in Kodi itself, It worked in Alpha1 and then didn't in Alpha2. I'm trying to lock down exactly when so I can try to get it bug-fixed by the dev team.
Any other bugs?
Using a NUC7PJYHN and a 2820FYKH0 Intel NUC running LibreELEC, and two FireTVs.:)