When you group 2 fields and then try to group another field to them, instead of grouping all three of them on the same level another inner grouping of the last two of them created. This used to work in the past. Videos before words - you can see it in the attachement.

More generic description: when you add 3 components in one row the layout gets broken. This is problem both in form modeler AND in Content Management's Perspective Editor. The problem is not only that the layout is broken, but that after saving and closing the form which was broken in this way, it's no longer possible to open and fix it.