Pipeline Start and End Period failing though REST API but successful when executed manually.

Summary:

Pipeline variables for start and end period work properly when executed manually but fail whenever the job is executed through a REST API call.

Content:

I've created a pipeline integration that use the pipeline variables for start period and end period:

Which are used in the integration which is specified in the pipeline:

Whenever I trigger this pipeline manually, everything works and the start and end period are correctly used through the variables. I verify this using the logfile from the Pipeline Job:

Whenever I try to execute this pipeline job through a Groovy script which calls the REST API the job is triggered but the variables which store the start and end period are incorrectly applied: