My sincere apologies for that bug. When a job definition is saved, it erases all extensions from the copy of the job on disc. It doesn't erase the extensions in memory, just on the disc. That makes the bug even worse, since the user doesn't realize their job definition was damaged until the next restart of Jenkins. I introduced it in git plugin 2.4.3 that was released 19 Mar 2016. It was reported as JENKINS-33695 and JENKINS-33811 and JENKINS-33811.
It is fixed in git plugin 2.4.4 that was released 24 Mar 2016 (about 8 hours ago). The plugin is not yet visible in the update center, but should be visible within 24 hours. If you need the fix sooner than it is available in the update center, you can download it directly from artifactory and upload it to your Jenkins.
There is now an automated test in the git plugin which checks for that bug, and I have a separate end to end test in my Jenkins docker instance which checks for that bug.
Thanks,
Mark Waite