Re-inspecting the images you provided, would it be more likely that they built up a flat base with concrete, then poured the slab over top, or did they inject/fill under the slab at a later date. Injecting under pressure indicates what you spelled out - that they may have inadvertently filled or blocked lateral drain lines at the point where the failure took place.
But even if they pre-poured concrete to level the ground prior to laying down the deck, did some of the weathered rock around the concrete pour wash away and cause the drain pipe to fall/move so that it was no longer piping water to the side drains but further undermining the area.
It’s pretty evident either way that that chunk has 2 separate pieces (perhaps anchored together even).
Half deducing, half speculating here.
Here are some points to further explore the evidence.
(1) If it was a pour to level to grade, then there should be anchors protruding from the bottom of the pour. This is how the blueprint designs identified "concrete to grade". Yet there is no sign of any protruding anchors.
(2) The construction photograph reveals that there was an aggregate layer compacted and leveled "to grade" before the concrete slabs were poured. Strong evidence is revealed in the diagonal "percolation seam" (i.e. if it were concrete in the picture, the seam would not be diagonal, plus the color hue does not match concrete coloring of the keyed chute blocks).
(3) This infers that there was a layer of aggregate & fines under the original slabs. The BOC confirms this in their first report of "filling" material between the subrock and the slabs.
(4) A smooth layer of aggregate and fines would be a much better suitable foundation to lay the frangible (fragile) Vitrified Clay Pipe Drain upon. Otherwise, laying the VCP pipe on rough rock or even fill grade concrete would risk cracking of the pipe. (5) IF sections of "fill grade concrete" were present, the construction photograph does not reveal this. You should see areas of discoloration. The image is uniform in hue up the main spillway. HOWEVER, the image does not show the final upper sections of the Upper main spillway. That is where archive documents identified "less than competent rock" where they had to resort to "chain link", "pigtail anchors", and rock bolts. This could explain the larger sections of potential coring depths of concrete in the upper main spillway.
(6) Then the key puzzle question is the seam of the upper slab section mating with the lower larger block of concrete. It is very clean. If there were the "aggregate & fines" that provided the original base to the slab pours, where did it go? Most likely it was washed away. OR the slab pour was able to penetrate the aggregate with the fluidic part of the concrete grout to bond with the "fill to grade concrete" below. This would make the bond junction less competent and more likely to split away from the forces that transported the chunk down the spillway.
(7) There is a telltale "hole" in the slab that looks to be where a drain pipe was located. The hole looks too small for a normal sized drain. But the hole could be an artifact of the drain pipe falling down & into a "fill void". The hole may be the leftover of the polyethylene plastic & some drain rock trapped in the slab pour grout.
(8) It is clear that the longitudinal drain was completely inoperative at the east sidewall drain near the failure area. Either the drain was sealed or the longitudinal drain broke away & water flowed under the slabs down slope or flowed into a subterranean channel.
(9) The fact that DSOD inspectors found loud echoing voids from sounding reveals that "repairs" (known to have been repeatedly performed in the blowout failure area) "best fit" that grout material was being emplaced. The pre-failure images reveal a large seam void was cavitating prior to the blowout. We don't know if large "repair holes" were present at this area (evidence gone), but the BOC did state that this was observed in other areas of the spillway. A large "repair hole" would facilitate pours of concrete to fill a large void.
(10) Note: consider unconfirmed: What tends to tie some of this together is insider tips from people doing repairs reporting of 6 foot voids and 4 foot voids being filled. These tipsters have come out of the woodwork but don't want to go public. Thus, evidence has to be pieced together. Hopefully, the forensic team will not be inhibited and will investigate all clues available.