Local files iPhone but with a twist

[EmeraldShovel74]

Plan

Premium

Country

Netherlands

Device

Windows 10 PC and iPhone 7 (128gb)

Operating System

iOS 13.3

My Question or Issue

So after a while of actually thinking that my troubles with local files and iPhones were over, I got the following:
iPhone doesn't download local files when I make a new playlist for them (it just shows up as an empty playlist on my phone), but after some tinkering I found out that if I select and play them from my PC when I put the tracks in an older playlist of mine it will actually work (they will also work then in a new designated playlist but will disappear randomly into thin air), but is there a way to get my local files to work on my iPhone within a new playlist? It isn't the codec and I've reinstalled my Windows 10 Spotify already (not Microsoft store app).