if the script(s) run as expected with delete and fail with truncate and the only change is subing truncate table and delete script steps then i would put in a report Report a Product Issue
Only gotcha with truncate Table is that it does not respect cascading deletes.
The docs don't explicitly say that Truncate Table is limited by the delete permissions.. but it should if that is the case. i'm a big fan of verbosity in help documents.
Truncate also cannot be done from one file to delete data sourced from another file as you might encounter if using data separation.
So in case anyone missed it it over in the Report a Product Issue discussion, Full Access Privilege is required for this script step. This is outlined in the Note section of the FM 16 Help page (which I somehow missed) but that note is NOT currently present in the FM 15 Help page.
So making the script run with Full Access Privileges fixes my problem.