Locally Synced Music from Mac Disappeared Overnight – No Apple Music Subscription

I synced my music from Mac (Apple Music) to my iPhone last night (around 1 AM), and everything seemed fine – I could listen to it in the car and everything. But when I woke up this morning, I found that all the music was gone, and my iPhone’s music library was empty.

I don’t have an Apple Music subscription, I just synced local files manually. Has anyone experienced this? Why would my music disappear like that, and how can I prevent this from happening again? Is subscribing Apple Music the only choice?

Any help would be appreciated! Thanks!