I can reproduce this, you don;t need dicom. It definitely reproduces with sample MRHead data as you described.
I suspect this has something to do with saving an empty segmentation object. Because I delete the segmentaiton, and recreate it, threshold tool starts working again.
I think it does qualify as a bug. And I am not seeing any errors in the log file.
This is probably the issue, because a transient ‘preview’ of the threshold is not intended to be saved and restored. It’s sort of like a popup menu that you wouldn’t expect to be back when you reload the scene.
This probably is a problem, in that the state of the scene becomes inconsistent. I guess that save operations should trigger an even that the segment editor (and other modules) can respond to and put the scene into a consistent state.
Yes, I think it’s a corner case, but it should be tracked anyway so thanks if you can file an issue. @lassoan showed us an in-progress fix that he thinks would address this.