I have some results to share with anyone who has been reading this thread. It turns out, another employee working on the development side had placed a trigger on the site table that had something to do with the append failure. We removed the trigger and success! I'm still working on getting the details. From the surface it seems as though the trigger had some logic where "if x,y are null then". It seems that because some of the records are not geocoded due to bad address they have empty geometry. Not sure how exactly, but this played into the mix here. I will update with any future developments.