Why are my Music downloads Inconsistent on iOS 18?

I purchase a lot of music through iTunes. I have noticed a long term issue with downloading, which is inconsistent. That is, sometimes the song will automatically download onto my iPhone (iOS 18), sometimes I have to go in to my songs list and trigger downloading manually -- that can get very tedious. At times, I have downloaded and confirmed different songs, and they have "undownloaded" themselves.

I can't find any settings relative to this, but the experience I want is for my songs to download -- preferably automatically and at time of purchase.

I am often in areas with spotty cell coverage, my preference to have my music downloaded to my phone. That's all.

Is there a way I can fine-tune this issue? Is anyone else experiencing this problem?

[Re-Titled By Moderator]