For one of the freestyle jobs on our local Jenkins install, I clicked the "only show failures" link beneath the unit-test trend graph. This worked as expected. Clicking the "show test# and failure#" link does NOT bring back the default view.

I tried to test a bit locally what could be the cause and found this:

// In IE11 developer tools console:
document.cookie
"TestResultAction_failureOnly=true; TestResultAction_failureOnly=false; jenkins-timestamper-offset=-3600000; jenkins-timestamper=system; jenkins-timestamper-local=true; screenResolution=1920x1080"

So, there are 2 key-value pairs stored related to the setting of this graph, with an identical key. One is set to true, one is set to false. It seems setting the value goes to one of the keys, while reading the value uses the other key...