[Snake]

Unequipped (Notice the 73 SWA).


Equipped (SWA lowered to 53).


The possible reason is how Full Swing works:


This should be changed to "While you do not have a Shield or Sub-Weapon equipped in your off-hand". Just a case of DevLogic™ because you probably never expected that shields could go in main-hand. 

Tweak that and you'll safeproof the future where we can slam shields on people's faces.

[Autumn]

With no overbearing special effects and decently low base power, correcting this (if it is a bug) really has no major impact on anything. It did strike me as odd, but the wording seemed to pass so I didn't bug report it.

[WaifuApple]

Dev has addressed this before as not a bug, if I remember correctly. Which makes this more of a balancing thing.