There are two suggestions to try:
1. If you intend to play, you must place all the lyrics you have entered. If you don't place all the lyrics, click the Place Lyrics button, not that Play My Song button. That way you won't see the garbage that WILL appear. What you don't see, then shouldn't bother you.
2. Don White (thanks!) pointed out that you need to have 2 (not 1, but 2) blank lines at the end of your song. Do this by... pressing the Enter key 2 times after you last lyrics. While placing the lyrics, make sure you tap the spacebar twice for these two blank lines.
These two tricks have made each of my test songs perfect. The last words sweep correctly and I don't have the garbage line-overwriting-line that is in this and other Threads.
Let me know if this works for you all.
The "technical background". The graphics rendering code takes the text (ASCII) characters that you have placed to the music, and converts them into the RW codes that display as the CDG format. This is the most complex area of all of KHP's code. Given that it is drawn into a scaleable, resizeable, movable window makes it even more difficult to rework. As this thread and others point out, there are problems in Camelot (or should I say KHP). However, to fix them requires rewriting and retesting the code.... we're talking more than a few months here. Thus, we must have work-arounds to overcome these problems for now.