I have a bug which looks similar to JENKINS-14899:
Sometimes, when a job runs for a long time without anything getting output, the jenkins page stops updating until the job finishes. Then it add the completion lines (SUCCESS or FAILURE) to the job.
The difference is that the number of line does not matter (last bug occurence was on a job that output 264 lines (+ an unknown number of non-output lines)

This results in a truncated /console, /consoleFull and /consoleText.

All files do end with the two lines (if it succeeds):
Notifying upstream projects of job completion
Finished: SUCCESS
but hundreds of lines didn't get stored by jenkins (over 10 minutes of logs for my last job!)

I can often see this bug when I'm running a long impala query, where the output stops before the status line (something like "Returned 0 row(s) in 1987.42s") and the rest of the console logs never get output.

This happens approximately one time out of 20 for the jobs I'm running myself on this Jenkins server.