Lowe's Style Selections Armstrong Lock & Fold/SwiftLock 3/8" engineered hardwood floated over lightweight concrete.

My first experience with this system (and with finish flooring!), so I initially didn't know what to do about underlayment. I tried a few rows with Pergo Gold first, and rejected and returned it. Discovered MP Global QuietWalk, and am using that. (I didn't know at the time about Armstrong's recommended own Quiet Comfort underlayment.)

Is a little bit of discernible deflection in the lengthwise seams customary for any standard installation of the type of wood floor I have?

Are you getting play between the boards on the butt-joints? There should be none.
The underlayment does give, even when the whole floor has beed covered. This gives some level of comfort when it is walked on.