Local Files disappears from Playlists

[PerpetoosNoo]

Hi everyone!
I apologize for my bad English. I have a problem, since many bands aren't present on the platform, to always listen to them in the car I decided to put the songs of these bands on my phone and on my laptop so that I could insert them as local files. I create Playlists for each album and then I put my heart to all the songs so as to have them inserted together with all the others of the other groups present on Spoti in the maxi playlist "songs you like" (I don't know how it appears in English). The problem is that after 2 or 3 days of listening to them, they remain present in the "songs you like" maxi playlist but little by little they all disappear in the various Playlists. Anyone know why? Now I'll try not to put the heart and see if they remain...

Thanks at all the people who answers!

Luca