I have raised this problem, or similar to, some time ago and support admit there is a problem.
Noel has also been looking at this for me and I sent him some files where it was not possible to delete the lyrics.
In my case, the problem is with files which have line-based lyrics. You can use the functions above to delete them but the line-based lyrics remain, and you get a message advising of their existence every time you open the file.
You can still add note-based lyrics but the line-based lyrics remain.
I would really like to know how the programmers are going in trying to remove this bug. I suspect from what Noel says that it still remains in the 2017 version.
This is the topic I raised earlier:
http://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=368626#Post368626