We make extensive use of multi-line build descriptions.
E.g. inside one of our projects, we might have something like:
echo "BUILD DESCRIPTION IS Branch: $GIT_BRANCH<br>$FOOBAR_DESCRIBE<br>PRODUCTION: $PRODUCTION"

Then, in the Set Build description area, for Regular expression we have:
BUILD DESCRIPTION IS (.*)

On the Build page for any individual build, this will correctly be rendered as, for example:

Branch: refs/remotes/origin/grunt-copy-production
v3.3.1-20-g7494bddb
PRODUCTION: true

But on the Project Page, where the builds are listed, the <br> are no longer rendered. See attached screen shots.

This is true of the new (black) Jenkins UI. In the old (blue) Jenkins UI, the <br> would cause the multi-line description to be rendered on multiple lines.