That should not be how it works. If a department is lets say, 60 hours over, then the heatmap will flag up a number of things like the hours/shifts concerned then factor in things like availability, attendance etc. If you personally score high you have nothing to fear, they won't move a good reliable person on a flexi from a department if they score high.
Its those who score low that they will go for first but as I've already said. The heatmap is now a tool to lure people off fixed contracts and onto flexis, its the fixed crowd they are going after.