Do any other @YouTube users have the problem where if you put a video into a playlist while uploading, it simply does not go there? It shows its there on the back end, but isn't in the playlist. You have to uncheck them, save, recheck, and save again. Happening for years.
Sorry to hear you've been experiencing this issue for a while now, have you tried reaching out to our Creator Support? They're the best team who can assist you with your playlist issue. Here's how: https://yt.be/help/partners