v2.1.2 Build 16516.2700229
Detailed description: Wood Elf Lords, including Orion himself, can not upgrade Great Stag Knights with Swift & Sturdy in the red skill line. I have two lords, Orion and Dhanas, that have Great Stag Knights, the red skill line Swift & Sturdy, and a mix of rank 6 and 7 or higher units. The Weapon Strength, Melee Defence, and Charge Bonus are all identical between the units besides the minor difference from ranks gained.
Reproduce by leveling a lord down the red skill line and have rank 7 great stag knights in the army.