Okay, so I updated Slack and the problem was resolved by your hack of not allowing sorting searches :( But I'm still submitting because I put all this time into writing this up before trying an update.
Spotify (an old Desktop version that still allowed sorting searches)
macOS High Sierra (version 10.13.6 (17G65))
STEPS TO REPRODUCE:
1. Use the search bar to search for a string (e.g. "i don't care")
2. In the "Songs" section click on "SEE ALL"
3. Sort by a column (e.g. "ARTIST")
4. Drag a song to a playlist (e.g. the third song, "LAND OF THE FREAKS" by 2Chainz)
5. Go to the playlist
EXPECTED: you will see the song you dragged (e.g."LAND OF THE FREAKS" by 2Chainz)
ACTUAL: you will see a different song (e.g. "I Don't Care" by Nav, Metro Boomin)
This also worked for selecting multiple items
This also worked when right clicking and choosing "add to playlist"
APPARENT CAUSE: it appears that this is happening because an index is assigned upon the original search, and then moving songs does so by index -- meaning when I select the third song (sorted by artist) the song actually moved is the thing song (sorted by the default search sort).