Lookup Formula in Power Query

If I understand you correctly, paste the code below into the Advanced Editor. It seems to work with your data sample.

Read the code and comments to better understand the algorithm.

let

//change next line to reflect actual data source
    Source = Excel.CurrentWorkbook(){[Name="Table3"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Parse", type date}}),

//add index column to retain original order
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),

//group by "Emp"
    #"Grouped Rows" = Table.Group(#"Added Index", {"Emp"}, {

    //For each emp
        {"all", (t)=>
            let 

            //ensure date is sorted ascending
                sort = Table.Sort(t,{"Parse",Order.Ascending}),
            
            //shift segment down and up to compare this month to last month and next month
                shift = Table.FromColumns(
                    Table.ToColumns(sort) & 
                    {{null} & List.RemoveLastN(t[Code Segment])} & 
                    {List.RemoveFirstN(t[Code Segment]) & {null}},
                   {"Parse","Emp","Code Segment","Index", "Shift Seg Down", "Shift Seg Up"}),

                #"Add SegName" = Table.AddColumn(shift,"Segment Name",
                    each if [Code Segment] = ([Shift Seg Down]??[Code Segment]) then null else [Shift Seg Down]),

                #"Add Xfr Cnt" = Table.AddColumn(#"Add SegName","Transfer Count",
                    each if [Code Segment] <> ([Shift Seg Up]??[Code Segment])
                        then -1
                        else if [Code Segment] = ([Shift Seg Down]??[Code Segment]) 
                        then 0 else 1),

                #"Remove Shifted" = Table.RemoveColumns(#"Add Xfr Cnt",{"Shift Seg Down","Shift Seg Up"})
            in 
                #"Remove Shifted",
                type table[Parse=date, Emp=text, Code Segment=text, Index=Int64.Type,Segment Name=text, Transfer Count=Int64.Type]
                }}),

//Expand and sort the Grouped Columns
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"Emp"}),
    #"Expanded all" = Table.ExpandTableColumn(#"Removed Columns", "all", {"Parse", "Emp", "Code Segment", "Index", "Segment Name", "Transfer Count"}),
    #"Sorted Rows" = Table.Sort(#"Expanded all",{{"Index", Order.Ascending}}),
    #"Removed Columns1" = Table.RemoveColumns(#"Sorted Rows",{"Index"})
in
    #"Removed Columns1"