Two fixes.
Long playlists start playing straight away. Shuffling or playing a big playlist used to load every page of it before any sound came out, which on a few-thousand-track playlist meant a long wait staring at nothing. Playback now starts on the tracks already loaded and the rest of the playlist fills in behind it, so a shuffle is still a shuffle of the whole playlist rather than of the first page. YouTube Mixes get the same treatment.
Local files no longer offer YouTube Music actions. The player bar kept showing "Add to playlist" while a file from your own disk was playing, which let a local track end up in a YouTube Music playlist where it does not belong.