I’ll have to admit, I don’t understand “why” I have to do this. It works, but I don’t really feel like I’m learning in this instance.
Hi! Have a look at the graphs side by side. Forgetting the non-important changes in the visual aspects, these are not identical models. In the first one trend is shape 715x715, in the second one it is 715.
So these are doing fundamentally different things, and I suspect the first model was not doing what you wanted it do.
In terms of learning, I’d also suggest to really embrace the use of dims. I see that you’ve got partial use of dims at the moment… some of the plates in the graph digram have dimension names, but others don’t. My suggestion would be to use dims more (and move away from shape) and to really inspect the graphviz for correctness 
It can take a while to spot potential errors, but it is useful.
Hope that helps in terms of your frustration of not learning from that particular solution you found.