
It looks like that area was filled in with the Flat Map Failover due to lack of overlap in the treed area. Bare trees can be almost impossible to render accurately so a Flat Map might be a better choice there anyway. You can read more about that here:
https://support.dronesmadeeasy.com/hc/en-us/articles/360054457952-Flat-Map-Failover
The area that is shown in the web map but not in the regular TIF file is available in the Flat Map Patch TIF file which is also available for download. You would need to merge the two of those files to get them to show the same as they are in the web map.
You can read about how to do that here:
https://support.dronesmadeeasy.com/hc/en-us/articles/115003641246-QGIS-Raster-Merging

