Calculated column with the max per group

See the working here - Open a blank query - Home - Advanced Editor - Remove everything from there and paste the below code to test (later on when you use the query on your dataset, you will have to change the source appropriately. If you have columns other than these, then delete Changed type step and do a Changed type for complete table from UI again)

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Xc+xDQAhDATBXhyDZBsMfC2I/tv4k0g4pxPt7i2u7lVX1SFFTE55aIKcaYEa0wfqRKagYDLQYHLQZGqg9VBUtdQF8tQFaqkL1FMXKFJX3O3BNG/X+QE=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Date = _t, Rank = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Date", type date}, {"Rank", Int64.Type}}),
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),
    #"Added Custom" = Table.AddColumn(#"Added Index", "Highest Rank in a Group", each try if #"Added Index"[Rank]{[Index]+1}<[Rank] then [Rank] else null otherwise [Rank], Int64.Type),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Index"}),
    #"Filled Up" = Table.FillUp(#"Removed Columns",{"Highest Rank in a Group"})
in
    #"Filled Up"

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