Don't worry, Michael, I have no problem with this model.
It is only a test to understand what's happen.
I have other models with the same problem with completely different structure but for now I use a little trick:

1 - I look for the part that 'drive' the group
2 - Cycle thru select until it is the only selection (two or three clicks, strange but real)
3 - Click on 'separate' command'. Now the group is ungrouped but all the parts in the group (except the driver) are duplicate.
4 - Delete the duplicate parts.

Unlimited thanks
Gianrico