Adding data to a grid grouped by multiple columns gives unexpected result

[jeme]

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http://plnkr.co/edit/Hl140HS8fj2q71iz

In the example given by the Plunkr we have a grid which are grouped by two rows.
Note: The example given here is attempted to boil it down to it's essence, we do also have custom aggregators etc. But the example shows the same behavior, we also have an issue with aggregators on top of that but I think that might be linked to #4679 without having gone deep into it.

Whenever we add a new batch of rows, that is suppose to generate an entirely new group, then the grouping is messed up.
(Adding data is done by clicking the 'add data' button)

Instead of now having the groups 1 to 4, we now just have a single group (4) and the behavior under this is really odd in it self.
As a workaround I can manually clear the two groups and apply them again so I had hoped that clearing the groups and adding them again through the API would work but it doesn't appear to make a difference.