Calendar are not correctly calculated when creating a Forecast.
The execution may differ between Forecast & actual Workflow execution.
With the introduction of the Next Generation Forecast (v. 12.3) a new forecast model was developed:
https://docs.automic.com/documentation/webhelp/english/AA/12.3/DOCU/12.3/Automic Automation Guides/help.htm#_Common/ReleaseHighlights/RH_AE.htm#link8
During this migration to v12.3 some rarely used Job and Workflow configurations were not included completely in the forecast calculation.
Release : 12.3
Component : Automation Engine
Issue already being worked on by Engineering.
Fix will be provided in a future release (TBD).