Matt Finley suggested I might request a fix for this problem, which I raised at

http://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=262460#Post262460

In Editable Notation mode it's possible to shift a Section Letter horizontally, to avoid being overwritten by a chord name at the beginning of the same bar. Unfortunately, this horizontal setting is ignored, and the section letter and chord name are superimposed.

It's possible to work around the problem by using Section Text, which implies that the problem has been solved, just not applied to Section Letters.