@Roche I see under the progress title that we're viewing "All Tasks Excluding (no section)." This means that we're viewing all tasks except those that aren't listed under a section. Sections are header tasks such as:
I suspect that on March 5th (when the plateau happens in the graph) something was updated in this project involving sections. Have any sections been removed or have tasks been moved out of sections?
Regardless of the section updates, you can always experiment with the filters you're using in the progress view. Click the gear icon next to "Progress" and select different filters.
Let us know if that helps! If not, we'll keep on working on it.