I have the same problem, and I only have four measly playlists and every one of them has songs that I bought and are on my phone.
After updating to iOS18 they disappeared. WHen I connect my iPhone to iTunes (Windows version) I see all four playlists, but my phone just says "Subscribe to Apple Music to save music" when I tap the PLAYLISTS section.
I moved them back onto my phone on iTunes, and as iTunes could already see them this meant doubling-up an all for, then all of a sudden 8 appeared on my phone. Well, okay, I deleted the duplicates and all was well for a few days, then yesterday I opened my playlists again (having never connected to iTunes since the last time when I recovered them), and all four had vanished again.
This time I just connected my phone to iTunes again and hit SYNC and I see all four are back again. We'll see how long they last............