dials.scale deltacchalf filtering imageset_id error

[rjgildea]

dials.scale crashes at the very end with the following error, likely introduced as a consequence of #1763:

$ dials.scale /dls/mx-scratch/rjgildea/multiplex/pierre/Lysozyme/Au/multiplex/002/{models.expt,observations.refl} anomalous=True full_matrix=False filtering.method=deltacchalf filtering.deltacchalf.max_cycles=6 filtering.deltacchalf.stdcutoff=4
...
Total time taken: 155.6907s 
================================================================================
Writing html report to dials.scale.html
Traceback (most recent call last):
  File "/dls/science/users/hko55533/software/cctbx/build/../modules/dials/command_line/scale.py", line 258, in <module>
    run()
  File "/dls/science/users/hko55533/software/cctbx/conda_base/lib/python3.8/contextlib.py", line 75, in inner
    return func(*args, **kwds)
  File "/dls/science/users/hko55533/software/cctbx/build/../modules/dials/command_line/scale.py", line 231, in run
    scaled_experiments, joint_table = run_scaling(params, experiments, reflections)
  File "/dls/science/users/hko55533/software/cctbx/build/../modules/dials/command_line/scale.py", line 194, in run_scaling
    experiments, joint_table = algorithm.finish()
  File "/dls/science/users/hko55533/software/cctbx/modules/dials/algorithms/scaling/algorithm.py", line 311, in finish
    self.reflections = update_imageset_ids(self.experiments, self.reflections)
  File "/dls/science/users/hko55533/software/cctbx/modules/dials/util/multi_dataset_handling.py", line 388, in update_imageset_ids
    assert len(set(table["imageset_id"])) == 1, set(table["imageset_id"])
AssertionError: Please report this error to dials-support@lists.sourceforge.net: {0, 1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 23, 24, 25}