@WardenclyffeTower said:

Disney Junior considered Return of the Roar a movie when it first aired it and called the episode which is currently listed as season 1 episode 1 titled "Never Judge a Hyena By Its Spots" the series premiere.
Your Wikipedia link cites the DisneyABCPress.com website as one of it's two only sources, yet the contents of the article isn't supported by the source. That's why it's never good to trust a Wikipedia article and you should instead check the sources.

Surprisingly, the English Wikipedia article seems to currently have it correct: https://en.wikipedia.org/wiki/List_of_The_Lion_Guard_episodes#Pilot_(2015)

Here's an archive of the press website just before the series premiered. It states "Series Premieres January 15th" which is the air date of the episode titled "Never Judge a Hyena By Its Spots":
https://web.archive.org/web/20151231061536/http://www.disneyabcpress.com/disneyjunior/shows/the-lion-guard/episodes/

Disney+ didn't exist in 2016, so it's not relevant what they have listed there. Their streaming service at the time was watchdisneyjunior.go.com and archives of the website shortly after the series premiered also supports that Return of the Roar was a movie and "Never Judge a Hyena By Its Spots" was episode 1.
https://web.archive.org/web/20160112232054/http://watchdisneyjunior.go.com/the-lion-guard
https://web.archive.org/web/20160707050638/http://watchdisneyjunior.go.com/the-lion-guard

The Lion Guard: Return of the Roar can be found here: https://www.themoviedb.org/movie/359983-the-lion-guard-return-of-the-roar

Thanks for the explanation !