Insert previous rows with calculated columns

When your example does not reflect the variability in your actual data, and your description is unclear and incomplete, I am not surprised.

Given your new data, it is merely a matter of changing the custom function to account for more than one "AER" row.

The main function remains the same:

Custom Function

//rename fnInsertAERrows

(tbl as table)=>

let
    #"AER Rows" = Table.SelectRows(tbl, each [Mode]="AER"),

    #"All BER Rows" = Table.SelectRows(tbl, each [Mode]="BER"),
    #"BER Rows" = Table.SelectRows(tbl, each [Mode] = "BER" and [Assignment] <> null),
   
    #"Code Count" = Table.Group(#"BER Rows",{"Code"},{
        {"Count", each Table.RowCount(_), Int64.Type}               
                }),
    #"Code with Max Rows" = Table.SelectRows(#"Code Count", each [Count] = List.Max(#"Code Count"[Count]))[Code],
   
    #"Full Rows to Insert" = Table.SelectRows(#"BER Rows", each List.Contains(#"Code with Max Rows", [Code])),
   
    #"Modified Rows to Insert" = List.Accumulate(List.Numbers(0,Table.RowCount(#"AER Rows")),{#"All BER Rows"},(state, current)=>
    state & {Table.FromRecords({#"AER Rows"{current}}) & 
    Table.FromRecords(Table.TransformRows(#"Full Rows to Insert", (r) =>
        Record.TransformFields(r,
        {{"Mode", each "AER"},
         {"Cost", each null},
         {"Code", each #"AER Rows"[Code]{current}}})))}
         
    ),
   
    #"Insert the rows" = Table.Combine(#"Modified Rows to Insert")
   
in
    #"Insert the rows"

Main Function

let

//Change next line to reflect your actual data source
    Source = Excel.CurrentWorkbook(){[Name="Table22"]}[Content],

//set the data types
    #"Changed Type" = Table.TransformColumnTypes(Source,{
        {"ID", Int64.Type}, {"Code", Int64.Type}, {"Assignment", Int64.Type}, {"AssignmentDesc", type text}, 
        {"Cost", Currency.Type}, {"Mode", type text}, {"AllCost", Currency.Type}}),
    
//group rows by ID and aggregate using the Custom function
    #"Grouped Rows" = Table.Group(#"Changed Type", {"ID"}, {
        {"added Rows", each fnInsertAERrows(_), 
            type table [ID=Int64.Type, Code=Int64.Type, Assignment=Int64.Type, AssignmentDesc=nullable text, Cost=Currency.Type, Mode=nullable text, AllCost=Currency.Type]}}),

//Expand the resultant tables
    #"Expanded added Rows" = Table.ExpandTableColumn(#"Grouped Rows", "added Rows", {"Code", "Assignment", "AssignmentDesc", "Cost", "Mode", "AllCost"})
in
    #"Expanded added Rows"

Source Data (your modified data in last post)

Results

The code would be pasted as separate queries into the Query (Power Query) Advanced Editor.

Please read the comments in the code for important changes you will need to make in the pasted code and query name.