why does music on ios delete songs loaded from my macbook

First: I hate the new 'music' app. Compared to the itunes+ipod/iphone system that existed before, the new music app has blatantly removed any functionality and convenience based around using my own music, likely in an attempt to annoy me into paying for a premium 'music' subscription. It's really become complete trash. Apple, PLEASE go back to my phone being usable as an ipod. wtf, this was in the orignial

So at any rate, I jumped through the hoops of plugging my iphone 12 directly into the macbook, using the jank UI in 'finder' I selected the playlists I want synced to my iphone, and waited for it to transfer. That worked fine for a while, I was able to listen to my own music on headphones. But several times I've opened up 'music' and ALL of the music files I loaded from my macbook are missing. I've had to do this process three times, waiting for all of the selected music to transfer each time.

Why is 'music' for ios deleting music files from my device?