If you remove a large tree, you had either best remove ALL, and I do mean all, the roots system or you will be dealing with cavities left by rotting tree roots for decades.
They did not remove anywhere near enough root system and you are simply dealing with a collapsing root system.
If you do not dig it out entirely what ever you put on top will continue to fall or sag for years.
It is moist because rotting roots hold moisture like a sponge.
I am living part time in a 100 year old house that has had many elm trees removed over the decades.
I put paver stones over one spot, a driveway before I reworked it where the former owner said he thought a big elm once stood and no longer used it as as parking spot.
Now the pavers are falling in spots and I have to figure out how much of a root system is falling.
Any tree that was there died over sixty years ago.
Although this is better than a spot on the opposite side of the house where either a former well, or out house, spot in the garden I had one morning left a hole approx. 6 feet across and well over 6 feet deep.
