When in the left panel (Library filter > Playlists) you click on the Playlist Explorer entry, the playlists that were present there, get moved to a new large vertical panel besides it, and your Left Main Panel gets disabled.
Suppose you had the tab 'Inbox' open. (unlocked)
That tab will change to 'Playlist Explorer' and the panel will display your playlists.
If next you want to revert to the previous situation, you could click the 'Back' icon on the top left.
But then the playlists that were moved to the large vertical Playlist Explorer panel, will not re-appear back in the Left Side Panel under Playlists.
And perhaps add the feature that when you have opened the new Playlist Explorer panel and you want to close it and revert to the previous situation, you can do that in the same way:
By clicking the Playlist Explorer entry in the Left Main panel again.