Elastic Mapreduce Map output lost
There has been some development since this question was asked. The question was discussed in more detail in jira MAPREDUCE-2389.
A key part in this jira, and any other jira that was linked here or there, is that the issue was linked to the jetty version. At the time it was not possible to go to a newer jetty version.
By now the referred version is long deprecated, and as such the situation should be fully resolved for everyone.