I have noticed before that some songs which I was sure I had liked (little green heart) before have somehow got unliked by spotify. This only happened a few times before so I brushed it off as a minor bug, however today, as I was going through a lot of playlists and artists I noticed VAST amounts of songs which I 100%, without a doubt had liked before, were now unliked (blank heart).
I'm not sure what caused this or if this is a bug, but it would be nice to fix this as I had to manually go back and try to find all the songs that may have gotten unliked by spotify.
Thanks for your replies and the info you've shared.
Songs can be removed and replaced on Spotify depending on the songs' right holders/specific distribution agreements with Spotify and when re-uploaded to the platform they'll be a different song compared to the removed/replaced version if the related metadata were edited (which is most likely the case if a song gets replaced with a different/newer/updated version) despite the title staying the same. This could be one of the reasons some of your songs are disappearing from your Liked Songs.
Another reason might be broken cache between multiple devices. This can occur on very large collections with more than a thousand songs in them. Especially if you're using your account across platforms. Best way around this is to occasionally clear the app cache on all your devices. Another thing worth trying is to split your Liked songs into playlists, this will also increase the app's performance when loading on some devices.
Also, for anybody having this issue only on mobile - you can check out this Ongoing Issue thread. Add your +Vote and subscribe to stay tuned with any updates on the matter.
Hope you find this info helpful.