Official comment
Due to the lack overlap in your data collection much of the area was unable to be stitched using photogrammetry so the Flat Map Failover was used. To process large areas of trees like that you need to use 80% overlap or more. Only the lower portion has a 3D result. The rest can be downloaded as the Flat Map Patch GeoTiff. 
You can read more about this here:
https://support.dronesmadeeasy.com/hc/en-us/articles/360054457952-Flat-Map-Failover
https://support.dronesmadeeasy.com/hc/en-us/articles/360054461812-Flat-Maps