When GCPs are located under vegetation, such as under a forest canopy, it's recommend to enable "Compare GCPs only against ground classified points" in SpatialFuser:

This will ensure that GCPs are compared to a TIN surface made from only ground points. If this setting is not enabled, a lidar surface created from all points will be used to compute GCP delta-Z values, which will likely cause erroneous lidar delta-Z values. If GCPs are marked as Control points, they will be used to compute a rigid adjustment of the pointcloud based on these erroneous delta-Z value, which will further complicate the issue.