When working the Revit MEP tutorial, I continued from the Resolving Conflicts tutorial to the Sizing Primary Duct tutorial, using the same drawing. Sizing the Level 1 duct worked just fine. However, on Level 2, Revit reduced the primary duct sizes in both directions from the middle. This happened on both systems. To make sure I hadn't done something wrong in the earlier tutorial, I opened the file intended for use with the duct sizing tutorial. Same result. Revit sized the Level 1 duct correctly, but sized the Level 2 systems with the largest diameter in the middle, reducing in size toward both ends.
Yes, I inserted the endcaps as instructed. No, I did not insert endcaps on both ends.