The "File exists" condition was successfully evaluated (to false)
The result of evaluating a Run condition is binary. The "On evaluation failure" allows you to choose what happens if the condition failed to discover whether the file exists or not.

If you want to mark the build unstable when the file does not exist you can add another conditional action, and select "Set the build status" from the Fail the Build Plugin https://wiki.jenkins-ci.org/display/JENKINS/Fail+The+Build+Plugin

If I understand correctly:

The run condition is true: action is executed
The run condition is false: action is NOT executed
The run condition cannot be evaluated: action is determined by the choice made by "On evaluation failure"

Now, after I download the Fail the Build Plugin, I can't use it with the Flexible Publish plugin as it is not in the list of Actions. I have attached a new image.

Yes, that is correct.

Oops, sorry - Fail the build is a "builder". You can use it by installing the https://wiki.jenkins-ci.org/display/JENKINS/Any+Build+Step+Plugin and enabling "Any build step" for the Flexible publisher in the main Jenkins configuration.

It's a bit convoluted, but it works now. The conditional action is a shell that checks that the file exists, and if it does, it returns non-zero. This way if the file does not exist, the action, which is to set the build status to unstable, is executed. I wish there was an easier way.

You are correct, there is no bug with the flexible publish plugin.

Thanks.

Thank you!

You dont need a shell - use the "Not" condition with the "File exists" condition and the action is "Set build status".