Running Total start over

Date Expense Date Expense RTSO
01/01/2020
1
01/01/2020
1
1
02/01/2020
2
02/01/2020
2
3
03/01/2020
3
03/01/2020
3
6
04/01/2020
4
04/01/2020
4
10
05/01/2020
5
05/01/2020
5
15
06/01/2020
6
06/01/2020
6
21
31/01/2020
7
31/01/2020
7
28
01/02/2020
8
01/02/2020
8
8
02/02/2020
9
02/02/2020
9
17
05/02/2020
10
05/02/2020
10
27
02/03/2020
11
02/03/2020
11
11
03/03/2020
1
03/03/2020
1
12


Code:
// Table1
let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    Month = Table.Sort(Table.AddColumn(Source, "Month", each Date.Month([Date]), Int64.Type),{{"Month", Order.Ascending}}),
    TableType = Value.Type(Table.AddColumn(Source, "RTSO", each null, type number)),
    Group = Table.Group(Month, {"Month"}, {{"AllData", fnRTSO, TableType}}),
    Expand = Table.ExpandTableColumn(Group, "AllData", {"Date", "Expense", "RTSO"}, {"Date", "Expense", "RTSO"}),
    TypeDate = Table.TransformColumnTypes(Expand,{{"Date", type date}}),
    TSC = Table.SelectColumns(TypeDate,{"Date", "Expense", "RTSO"})
in
    TSC

// fnRTSO
(MyTable as table) as table =>
let
    Source = Table.Buffer(MyTable),
    TableType = Value.Type(Table.AddColumn(Source, "RTSO", each null, type number)),
    Cumulative = List.Skip(List.Accumulate(Source[Expense],{0},(Cumulative, Value) => Cumulative & {List.Last(Cumulative) + Value})),
    RTSO = Table.FromColumns(Table.ToColumns(Source)&{Cumulative},TableType)
in
    RTSO