These issues are not actually connected to Spotify or Spotify's fault. Actually, the content is sent to Spotify by labels and services that only have the opportunity to label it explicit or not themselves. Spotify can't change this type of information without the content first being sent with the correct data from the sources.
However, if you come across music that should be labelled with the Explicit data tag, you can report the song to the following link:
Click on "I want to report a broken song or wrong song information". Click on the Green Button that says "I STILL NEED HELP". Copy and Paste the track URI links to the songs into the correct field. Type the description of the issue you have to the "Tell us more" field. Click on the green button "SEND QUESTION".