Is anything supporting the 'spine' of your 'box spring' type foundation?  I'm talking about that wood slat that runs from top to bottom in center at bottom of the foundation.  If it's a Queen box spring, it may be supported by a center bed frame rail.  If a Full, Twin or two halves of King, I'll bet that spine is not supported.  Those spines flex down.  The foundation surface flexes down.  Surface of the mattress can flex down or sag.

You may not notice unless you put some kind of straight edge against that center slat.

I'll bet many foundations are not fully supported in the center which contributes to saggy mattresses.

I have a metal frame with center support.  I even had to add 1/2" plywood to fill in the gap between the support and spine of foundation.

It seems kinda sad there isn't a standard between frames and foundations to fully support not just the edge of foundations, but the center support slat.  After all, who sleeps on the very edge of the bed, ready to fall off?