Wednesday, March 11, from around 7:05 p.m. (GMT+1). Newly played on Mac: Sasha Berliner: Onyx (Tidal). The last entry, Muncie Girls: Fixed Ideals (Local File), remains, but Maggie Rogers: Don’t Forget Me (Tidal) disappears from the middle. Same result on iPhone, iPad, MacBook.
Play Sasha Berliner: Fantôme (Tidal) and everything works correctly. The last entry, Muncie Girls: Fixed Ideals (Local File), disappears.
Play Aaron Goldberg: At the Edge of the World (Local File). Everything works correctly, including synchronization.
Play Jenny Hval: Iris Silver Mist (Local File). The last entry, Ratboys: Printer’s Devil (Tidal), remains, but The Last Dinner Party: From The Pyre (Tidal) disappears from the middle. However, this does not happen on the iPad, iPhone, or MacBook, where the last entry disappears and The Last Dinner Party remains (as it should).
Play Aaron Goldberg: The Now (Local File). The Beths: Straight Line Was A Lie and Hayley Williams: Ego Death At A Bachelorette Party (both Tidal) have disappeared. However, this is not the case on the iPad, iPhone, or MacBook, where the lists are correct.
Approximately 9:31 p.m. (GMT+1). I’m playing Muncie Girls: B-Sides The Point (Local File). Suddenly, entries that had already been deleted reappear at the end of the list: Steve Reich: Music for 18 Musicians, Ratboys: Printer’s Devil, Muncie Girls: Fixed Ideals, Brian Eno & Beatie Wolfe: Luminal (all Local Files) + The Beatles (as artist). And there are only 98 entries left. None of this is on the MacBook, iPhone, or iPad, but after restarting the apps, the same problem occurs.
Play Muncie Girls: Fixed Ideals (Local File). Suddenly, there are only 95 entries left in the list.
So it’s quite a jungle, sometimes this way, sometimes that way, and without any discernible pattern. And everything is a bit difficult to document. 
Edit: currently only 90 entries left on the list.