I noticed that the steps going up to the bedroom in our fifth seemed to flex more than they should, so went into the basement and removed the rear "wall" to access the underneath of the stair structure. And of course, there was indeed an issue.
The structure itself is sturdy enough, heavy plywood sides to which the steps are stapled to. But, they are "stapled", and it seems that FR couldn't afford enough staples to do the job properly. So I drilled pilot holes, careful to not drill though the top of the steps, and put about 10 screws in each step, two in each end and 6 across the front of each step.
I only needed to do the top 2 steps, as the bottom one is reasonably supported.
Now the steps are very solid and don't flex at all.
I wonder what else I will find?