I've upgraded from Jenkins 1.653 to 2.36 and P4-Plugin from 1.4.2 to 1.4.12 today and immediately started facing this issue, which broken all my CI build infrstructure, since my bash scripts relay on number provided by P4_CHANGELIST variable.

The issue is that it stores "now" instead of revision number if SCM is synced to head:

As I understand this is only when "Pin build at Perforce Label" is set to "now", e.g. via a Parameter variable.

From build console output:

P4 Task: syncing files at client/label: now
...
+ echo P4_CHANGELIST
now
...
Set build name.
New build name is 'now'

Previously was:

P4 Task: syncing files at change: 171134
...
+ echo P4_CHANGELIST
171134
...
Set build name.
New build name is '171134'

Is it some new design or a bug?
If it is new design intent, then this makes P4 plugin almost unusable for me...

I suppose this may have changed/been broken during changes made for env vars to work in Pipeline plugin...

Not sure at which version it is started, going to downgrade and experiment (reverting back to 1.4.2 helped to return to previous behavior).

p.s. refer to comments for more details found during a bit deeper investigation of this issue...