Hi @FredHa,
As Designed, there are certain areas of Workfront that calculate numbers of interest in memory, at that point in time, to ensure they are current and accurate...but the downside is that by not persisting such data, they are then ineligible for native reporting.
Given that, in this case, I invite you to consider our Capacity Charts, Heatmap, and AFA Burn Report solutions (among our other Resourcing solutions), in case they meet -- or ideally exceed -- your requirements.
Regards,
Doug