Get End Date by Next Action Date - Get Continuous Date Range by Category

Based on your description, I created data to reproduce your scenario. The pbix file is attached in the end.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("nZS9bsMgFIVfxfKcUi4/BuZ06dQqaqcoA0qsxFJlRzbJ0KcvEONS1TF2JA8gnQ8u59zr7TYHQhkv8lUuKXBM7IJwhAkiGKjdbMqnt/ao6+pbm6qpnzdlZ9rL3lzaMt+ttnkM4gJh5kBuN+9NVzkiWzf1tWw7u/wPSIR5AD4q81Vm65Ouj79HEwWFVwKyn1VKX9O5aU2XmeaeHOB2MMFxJS/a6OzzfNBmpHaBsHIEzCVAInA2ETpFMMY4d8+T7FF72VJ7h5twyl5KCwj2SqcsJu0NchB9JSJpVriA9oHAWClSSqWU02MOTDxkUrioQIAdyCZfEsnneRoKs0751L1Tk0+PCN9Zf80aiy0KY+j10dgi5a0YmSomJnByMAghlFI/UD23OI8AgooMXgISGEvmtdZ7U11toQdPAIAQzlgpHh2vASR9Tuye7WLpIIrZ/7mgpH0+6fYqgHN/tuq7JTmLA2GdFbO6JiJSXbP7AQ==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Position ID" = _t, #"Reports to Position" = _t, #"Effective Date" = _t, #"Workforce Action Reason Description" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Position ID", Int64.Type}, {"Reports to Position", Int64.Type}, {"Effective Date", type date}, {"Workforce Action Reason Description", type text}}),
    #"Filled Down" = Table.FillDown(#"Changed Type",{"Position ID"}),
    #"Grouped Rows" = 
            Table.Group(#"Filled Down", {"Position ID"},
                        {{"Count", (x)=>
                               Table.AddColumn( Table.AddIndexColumn(  x,"Index",0,1),"nextDate", each [
                                a=List.Buffer( x[Effective Date]&{Date.AddDays( List.Max( x[Effective Date]),1)}),
                                b=   List.Range( a,[Index]+1,1){0},
                                c = Date.AddDays(b,-1) ][c]    
)
 

}}),
    #"Removed Other Columns" = Table.SelectColumns(#"Grouped Rows",{"Count"}),
    #"Expanded Count" = Table.ExpandTableColumn(#"Removed Other Columns", "Count", {"Position ID", "Reports to Position", "Effective Date", "Workforce Action Reason Description", "Index", "nextDate"}, {"Position ID", "Reports to Position", "Effective Date", "Workforce Action Reason Description", "Index", "nextDate"}),
    #"Grouped Rows1" = Table.Group(#"Expanded Count", {"Position ID", "Reports to Position"}, {{"MinDate", each List.Min([Effective Date]), type nullable date}, {"MaxDate", each List.Max([nextDate]), type nullable date}},GroupKind.Local)
in
    #"Grouped Rows1"