Currently using 2.1.58 and have the same issue
Paul,
Try moving the DSM out of the root file, wherever that may be stored. From my understanding volumes are measured on the DSM once its generated instead of the point file…or process only steps 1 and 2. You should be able to delete vegetation, and then recalculate more accurate volumes.
This was the solution for me when I had overhanging conveyors.