Currently, a playlist is order from oldest-to-newest added track by default. Sorting a playlist by date added descending is only a visual change - the playlist order is not actually changing.
Despite sorting a playlist by clicking the column headers, such as by “added”, which appears to save or otherwise persist on the playlist, when playing the playlist from an external source such as via Google Assistant, or by clicking the play button on the playlist’s tile on the home page, the playlist is always played in the “real” playlist order -- first added, first played.
The expected behaviour would be to honor the stored sort order on the playlist. This only happens if you play the playlist using the “listen” button from inside the playlist itself. This really should be a global thing.
As it stands my only course of action here is to manually reverse the order of my playlist, and each time I thumb up a song, manually push it to the top of the playlist. The playlist is 116 tracks long and I have another 300 or so that will eventually be on it. This is untenable.