That maybe the case if it's just in one direction, it will not be just going up and down, all airframes will flex, and not just in one direction, torque from the engine will twist it one way, pressure from vert stab and rudder another, landing yet another again and so on, so the weakest point will be on the gusset at just bellow top longeron where the vertical brace starts and just above bottom longeron where the same brace ends, any sideways flexing here may or could fatigue over time regardless of allowable load limits. Just my thoughts.