As pointed out in the thread
http://lists.gnu.org/archive/html/lilypond-user/2014-04/msg00540.html
it is necessary to use \grace for the old grace note sync issue.
This commit makes that explicit in the known issues section in the NR
>>
@end lilypond
+Please make sure that you use the @code{\grace} command for the
+spacer part, even if the visual part uses @code{\acciaccatura} or
+@code{\appoggiatura} because otherwise an ugly slur fragment will
+be printed, connecting the invisible grace note with the following
+note.
+
The use of grace notes within voice contexts confuses the way the voice
is typeset. This can be overcome by inserting a rest or note between the
voice command and the grace note.