Power BI Grouped Rows

docs 

Hi, ideally you would do this before grouping the rows. but if you wanted to add after your last step, you can also use below logic. It's tested working on my end. 

let
    SplitNotes = Table.ExpandListColumn(
        Table.TransformColumns(Grouped Rows, {"Combined Notes", Splitter.SplitTextByDelimiter("#(lf)", QuoteStyle.Csv)}),
        "Combined Notes"
    ),

    AddDate = Table.AddColumn(SplitNotes, "NoteDate", each Date.FromText(Text.Start([Combined Notes], 10)), type date),

    Sorted = Table.Sort(AddDate, {{"objectid", Order.Ascending}, {"NoteDate", Order.Descending}}),

    Grouped = Table.Group(Sorted, {"objectid"}, {{"Top6Notes", each Table.FirstN(_, 6), type table [objectid=nullable number, Combined Notes=nullable text, NoteDate=nullable date]}}),

    CombineNotes = Table.TransformColumns(
        Grouped,
        {"Top6Notes", each Text.Combine([Combined Notes], "#(lf)"), type text}
    )
in
    CombineNotes