I need help determining whether a bookcase/storage cabinet is sagging and whether the it's being made worse by the solution my grandfather came up with.

So the bookcase/storage cabinet is about 8-10 years old and made from thick, heavy, particle board or composite board. It's from Sauder and it's one of their higher end products. The top of the unit had maybe 20-40 pounds of weight on it for over a year. In December, I did not notice the top sagging at all (this is something I would definitely notice). By May, my grandfather claims it was sagging so he put in something he rigged up that will keep the top from sagging. It's basically a bunch of blocks he inserted under the shelf to prop it up and maybe apply enough pressure to push the top shelf to try to reverse the sagging. Everything on the top shelf has been removed.

When my grandfather removed the blocks for 16 hrs, I did not notice the shelf sagging at all. Wouldn't this shelf have started sagging once the blocks were removed, even though the items on that top shelf had been removed?

How likely is it that 20-35 pounds of weight would've caused noticeable sagging within a 5 month period? The shelf should be rated for 35-50 pounds of weight. My guess is it's 50 pounds, the manual doesn't provide info on this.

The inside of this storage cabinet has a center vertical shelf that goes from the bottom to 3/4 up where a horizontal shelf sits upon it. Then there's about 8 inches or so of space between that horizontal shelf and the top. The blocks are placed in that space to keep the top shelf from sagging. It would seem to me that if the blocks are simply placed in there to prop up the top shelf, then it's no big deal. But if the blocks are placed in there to apply upwards pressure to the top shelf, that the blocks would also apply a downwards pressure to the center vertical shelf. That would then place pressure in the middle of the bottom horizontal "floor" of the cabinet, which could lead to sagging to the "floor" of the cabinet. Would that assumption be correct?