Mike Otto will have way more expertise in this area then I do but Ive moved a good bit of dirt for a living for 30 yrs, in your case the back side of the dam was not compacted adequately, it is settling and sliding all at the same time, all may not be lost but reality is it probably wont stop any time soon so filling the top is just a temporary fix till it settles, slides more.
the front of the dam may well be compacted enough to hold water just fine but inevitably the back side of the dam usually gets the least attention. as long as the dam is not leaking I would redo the back of the dam, key in the base and track it in a dozer width or two all the way to the top and then fill your top in properly.

All the really good ideas I've ever had came to me while I was milking a cow.