Help with identifying increase within a date range group

Although I am not quite sure why your Desired Output table is not following the rule 

1. Where/when the actual promotions are happening. (a promo is an increase in salary band),

you can give a try to the code below:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("ndJdC4IwFAbgvzK8Nraz2Ye3XUZSVHfhxbBBK9xgWtG/bzOhZfiVF8IOezznPXg8BkBZEAYrwdVkfcvQVmbcnGwFqH0lwsgS7cRdioc9EoYJYEogDtKwg7ry1uhcl1Irz1HS7zblWRh3IBjisQYwmX0MZZEtLoW68FwqtJdFpqsv9yZrk9Aue3pCcyV2zqljUDEWsWha0cNZ57xAO3mt1+Dls438KbtRd8JxDetwfzUcZH8avtcyc8/cGcPvskAJfz4Er+8ufEcxiT7pOl3c+GPYQPc9qJcufQE=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Personnel no." = _t, #"Personnel Name" = _t, #"Salary Band" = _t, #"Reason on Pay Record" = _t, #"Basic Pay Record Start Date" = _t]),
    Grouped = Table.Group(Source, {"Personnel no."}, {{"Gr", each let t=_ in Table.AddColumn( Table.AddColumn( Table.AddIndexColumn(t, "i",-1,1), "PROMO STATUS", each try if Number.From([#"Salary Band"]) > Number.From(t[#"Salary Band"]{[i]}) then "Promotion" else "" otherwise ""), "PROMO CYCLE", each if [#"PROMO STATUS"]="" then "" else if Date.Month(Date.From([#"Basic Pay Record Start Date"])) =3 then "On Cycle" else "Off Cycle" ), type table }}),
    Removed = Table.RemoveColumns(Grouped,{"Personnel no."}),
    FINAL = Table.ExpandTableColumn(Removed, "Gr", 
            List.RemoveItems (Table.ColumnNames(Removed[Gr]{0}), {"i"}))
in
    FINAL

Result: