I see two issues affecting this.
1. The pictures are extremely dark which makes it hard to identify and align details for stitching, particularly over homogeneous trees.
2. It also looks like th elevation may increase in the tree area (which are already 30 m higher than the ground) 30 m or so. That looks to be where it did not map well. Flying higher with 75/75 overlaps would help.
This helps explain what happens in areas like that.