Was there a change of tags between versions, Grouping <=> ContentGroup?

Yes, this change happened some time ago. In iTunes expose the fields Grouping and Work side by side to see where fields have the wrong values. Unfortunately the Work field isn't available to the iTunes for Windows scripting interface so I cannot write a script to remove/transfer incorrect values, but batch Get Info works.

tt2

It happened when the Work & Movement features were introduced in iTunes 12.5 a couple of years ago. See Organize your classical music in iTunes for an archived copy of the support document that came out at the time. I think they may already have been using the grouping field to link works together in the iTunes Store metadata (I recall seeing old listings for classical music in the store that showed in groups rather than a flat list of tracks) so chose that route regardless of the impact on end users.

tt2