Hi, just tested with iOS 11.4, but I haven't been able to replicate the issue. Lyrics synced just fine.
I used these steps:
1) Deleted all songs from my iPod Touch using MM
2) Selected one (not well known) track in MM and changed lyrics to 'aaaaa'
3) Right-click the track > Send to > iPod Touch
4) Opened music app, tapped the track to start playback
5) clicked track artwork below to bring the Now Playing screen with large artwork
6) Scrolled down to show the lyrics
=> Lyrics 'aaaaa' are there
Could you try with this steps or which steps did you use?
Note I used not well known track just to ensure that auto-search lyrics feature won't overwrite my 'aaaaa' lyrics.
Beta Testing for Windows Products and plugins