When putting credentials into a Folder credential store the P4 credentials cannot be "seen" by the jobs in the same folder. They have to be in the Global store to be seen.

The P4 plugin says "P4 Plugin credentials must be added to the Global or a user defined domain, using one of the two supported Credentials: 'Perforce Password Credential' or 'Perforce Ticket Credential'." (Emphasis Mine)

We need to have selective control of credentials per folder. This works for Subversion, but not for P4.