When I try to use a multi-line regexp in `font-lock-keywords', I have an
unusual result: some lines are highlighted and some are not (lines are
the same).

I have made a test mode, a text sample and a screenshot to show how it
looks for me when tested with "emacs -Q".

Recipe:
1. emacs -Q
2. Evaluate the contents of attached "test.el"
3. "M-x test-font-lock-mode" in a buffer with attached "sample"