I figured this one out investigating why "Changes" was always empty for each build, as well as the e-mail address recipient lists.

When using p4-plugin 1.8.3, the plugin will issue the following command when determining the changes :

p4 describe -s -m51 12345

The -m parameter is however not supported in our environment. As can be seen in the attached screenshot, the command returns :

Usage: describe [-d<flags> -s -S -f -O] changelist# ...
Invalid option: -m51.

The P4 documentation is quite confusing on whether it should be supported or not (compare : describe in 2017.2   and describe in 2017.2 ), and it also isn't clear to me whether this depends on the p4 client executable (I tried 2015.1, 2017.1, 2017.2) or on the server (in our case, v2012.1).

I found out that in all probability, the usage of the "-m" parameter was triggered by the changes for JENKINS-46671, released in 1.7.5 :

@22816 - Merge pull request #51 from jenkinsci/limit-describe. Limit output from describe. JENKINS-46671

After compiling and installing version 1.7.4 of the plugin, the error message goes away, and the Changes section gets populated properly.