With older Jenkins versions, the timestamp environment variable (formerly BUILD_ID) was
already set to the default timestamp format by the Jenkins core and was only modified by the Zen Timestamp plugin if the option "Change date pattern" was checked (inside the job or node configuration) and a different format was selected.
The new Zen plugin must be fixed to set the BUILD_TIMESTAMP variable in any case, even if there was no format defined inside the job or node configuration, by using the default timestamp format. This must be done by the plugin now, since BUILD_TIMESTAMP is not set by the core anymore.