I've also recently started noticing the lack of gapless playback when listening on my iPhone to tracks synced from my Windows PC with iTunes. I assumed it had to do with recently joining Apple Music, but now I'm not so sure. Tracks that used to be properly gapless on both iPhone and iTunes now have gaps.
As I test, I found one album that was still properly playing as gapless in iTunes. I deleted the album, restarted iTunes, and re-added the files from my PC. Suddenly the tracks were playing with gaps.
One thing I noticed - when you add files, iTunes used to pop up a little message saying "determining gapless playback information", but it no longer does so. Seems like this feature might have been removed with some recent iTunes update?
As I test, I found one album that was still properly playing as gapless in iTunes. I deleted the album, restarted iTunes, and re-added the files from my PC. Suddenly the tracks were playing with gaps.
One thing I noticed - when you add files, iTunes used to pop up a little message saying "determining gapless playback information", but it no longer does so. Seems like this feature might have been removed with some recent iTunes update?