I have just tested the iPhone 11 same version operating system and that has the same issue, the IMac software is absolutely fine,
It is hard to pick if you don’t sync your lyrics perfectly as it is almost half a step behind when you exit the program then return back to it.
I normally switch to different lines slightly earlier so the line is ready and steady for me when I need to sing, this I do consinstantly and very accurately but as I said before if I exit the program and re enter it throws the sync out by almost a whole line meaning that I start singing and then the line highlights.
But this is only on my IPad and iPhone but not on my iMac.
I have noticed this ages ago but never pointed it out as it is a small nuisance.