When using the WYSIWYG plugin to edit the contents of a call to the WYSIWYG plugin containing several consecutive line breaks, some of these newlines can be removed when saving.
This happens in Tiki 15, but did not happen in Tiki 12, as a result of r55910. This can be solved by removing the line added in r55910, as shown in the attached patch. This patch also solves issue #6591. But I've ended up only applying the changes to parserlib.php due to issues described in my third comment in #6591.