I have a similar problem with my S7. I have modified the Google Play Music setting to point to the SD card and I have reset the phone but it still only finds the music on the internal storage. Sorry, not an answer but resetting the phone did not help.
I was having the same problem on my Galaxy S7. Tried clearing the cache. No help. Went into Google Play and uninstalled Google Play Music then installed it. It found all the music on my phone (including SD card) and I now can see them in "songs" in the Google Play Music app.