I think you've found a gap in the FOW and Dip chart interaction.
To be clear -- whether the Pitt Resigns FOW event occurs, or the Brits drop the Prussian Subsidy due to economic concerns, the Prus will only suffer a -2 MW total.
I will have to make a Living Rules/Post Pub update on this. The big hit of Pitt Resigning historically was that Prussia lost the subsidy. The Prus might have lost it with Pitt at the reins anyway, given more negative economic conditions. So losing the subsidy only happens once, and losing the MW should only happen once as well.