P4 plugin : P4_CHANGELIST not available in workflow (pipeline)

I'm using p4 plugin for several months in freestyle jobs, and I'm moving to 'pipelines' implementation (workflow if you like)

According to this documentation, there are some known limitations :
https://github.com/jenkinsci/p4-plugin/blob/master/SETUP.md

especially :

No access to Environment ${VAR} variables

On the other hand, one line below is written :

Exposed Variables :

• P4_CHANGELIST - current changelist (valid for this to be blank when there are no changes).

Within my pipelines, what I see is that 'P4_CHANGELIST' is not available after a call to p4sync.

Is it expected ?

If so, how do I retrieve the changelist at which the sync is done ? (in order to propagate it to further steps and stages in the pipeline)