The Data Collection guidelines say to use at least 80% when trees are involved, so 80/70 isn't quite there. Bare trees are even harder to deal with due to the complexity and hard shadows. That middle portion where it falls apart is all bare trees.
When you ran the second map with just the southern portion it had the grassy areas to hold on to and keep things together. You can see that it started to fall apart up towards the north again though.
Bare trees can require 82-83% overlap to work well but even then it is going to be really lighting dependent for large areas like that.
We will grant a one-time 50% refund for overlap related issues.