There used to be a search bar when you dragged to the very top of a playlist (above the playlist logo) for you to search songs within that playlist, which has now been removed and replaced with a find song option hidden under the 3 dots menu in the top right.
I mainly use Spotify in offline mode and for some reason not all my downloaded songs appear when I search offline. It has took me 3 weeks to find the 'find song' option after help from the twitter account.
Bring back the quick and easy search bar!
Updated on 2019-03-07
Hey and thanks for your contribution!
A similar idea has also been suggested here
Add your vote and comments in support there please : )