Hi all,
I have been bothering by the same issue as well and have tried all the methods, e.g. check/uncheck, uninstalled/reinstall, etc.
Although the issue has not been fully resolved, I have found a trick which may temporarily restore the lyrics in your iPhone/iPad. Key tricks as follows:
1) Lyrics would not be properly shown if there is any "&" or special symbol in the lyrics. Remove/replace such abbreviation/symbol;
2) Leave the first line of the lyric blank & start the lyric on the second line.
After the above action, you may resync your device with itune and the lyrics should reappear in your device. Unfortunately, the above method could not completely resolve the bug problem in which some lyrics may disappear again for reset of device.
Despite not desirable, another method is to downgrade the iOS back to 9.x.
Hope Apple will fix the bugs ASAP, PLEASE