I have several jobs building via subversion commit triggers.
The commit messages contain several lines with the majority in the following format:

Commiter Name:
Details of change1.
Details of change2.

Up to Jenkins version 1.651.1 + Subversion plugin 2.5.7, the build page displayed the commit message with several lines correctly under "change history".
After upgrading to Jenkins 1.651.3 + Subversion plugin 2.6 and now Jenkins 2.7.1 + Subversion plugin 2.6, the commit message is displayed as a single line.
For example, the commit message above would be displayed as follows:

Commiter Name:<br>Details of change1.<br>Details of change2.

Checking the html page source of the build page, it looks like the newline characters in the commit message are being generated as:

<br>

As a result, the commit message is being displayed as a single line.