Bug identified with Proofpoint support. The handling of UTC vs localization doens't operate as expected, escalated to Proofpoint Dev.
Workaround, deploy to server running UTC system time (in case your org is crazy like mine and actually runs localized 😕 )
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Update from support:
In the script change line number 254 from servertimedt = datetime.now() to servertimedt = datetime.utcnow()
Thanks Mary. This code change should occur in a future version release to permanently correct this issue but your method is a valid work around for now.