I’ve probably missed something, but when I load a new playlist and a new track from it, the following track that automix selects is not from this new playlist, ( I think it’s actually loading alphabetically from the complete list of all tracks)
Which app version do you have - iPad, iPhone or Mac?
On iPad/iPhone: Click on Automix and choose a playlist for “Playlist”.
On a Mac: Click on the desired playlist to highlight it and then click on Automix -> Start Automix. The highlighted playlist will automatically be selected as “source”.