Please reference section 7.9.3 of the Development guide in the online help files for this:
When the process flow prompt value is a subvar that:
Does not include { } curly brackets
And the process flow is run:
With a schedule, awrun request, or REQUEST JOB condition
And the automation engine option:
In not checked
The prompt value is determined:
When the process flow goes to an INITIATED status
Please check the subvar in the process flow header prompt tab. I have seen after upgrades that if that subvar is enclosed in curly brackets we see this behavior. Please remove the curly brackets to see if it then exhibits the correct behavior.