Thanks for the update.
So how long and how much bad data was swept under the cracking concrete?
IF you are asking about post 65, the leakage on the backside of the earthen dam would be inferred from penetration of the compacted clay wall in the middle of the dam core. If a seam or crack formed in the clay water barrier, then this could allow "piping" of water from the pressure of the higher reservoir water through this seam or crack and then out the backside of the dam embankment.
It is normal to have some "weepage" in dams. However, if the source is not accurately investigated, this creates an unknown regarding a risk factor. What is surprising is that this seems that it was not investigated.
The wide area affected could represent a volume of water that is either weeping in a thin strata line, or could be a focused piping leak that widens into a strata line on the backside embankment. The latter issue is more dangerous as a focused piping leak could cause hydraulic erosion much quicker within the clay core - triggering an escalating situation.
So far, I've been able to get sat pic evidence of this condition back to 2013.