title: Columns types are gone after expanding table with Table.ExpandTableColumn in PowerQuery
body: <p>I was making a query, and once I finished it, I got lots of errors. After some exploration I figured out the cause of errors - after expanding columns from grouping (sum aggregation) all the columns types, assigned on previous steps, have gone completely. So here's example.</p>
  
  <p>Let's say, I have the following table:</p>
  
  <p><img src="https://i.imgur.com/vDZMQt0.png" alt="IMG1"></p>
  
  <p>PowerQuery query:</p>
  
  <pre><code>let
      Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
      ChangedType = Table.TransformColumnTypes(
                       Source,
                       {
                           {"ID", Int64.Type},
                           {"Date1", type date},
                           {"Sum", Int64.Type}
                       }),
      GroupedRows = Table.Group(
                       ChangedType,
                       {"ID"},
                       {
                           {"SUM_GROUP", each List.Sum([Sum]), type number}, 
                           {"ALL_ROWS", each _, type table}
                       }),
      ExpandedALLROWS = Table.ExpandTableColumn(
                           GroupedRows,
                           "ALL_ROWS",
                           {"ID", "Date1", "Sum"},
                           {"ALL_ROWS.ID", "ALL_ROWS.Sum", "ALL_ROWS.Date1"})
  in
      ExpandedALLROWS
  </code></pre>
  
  <p>On <em>ChangedType</em> step everything is OK:</p>
  
  <p><img src="https://i.imgur.com/CfZAeXC.png" alt="IMG2"></p>
  
  <p>But on <em>ExpandedALLROWS</em> step all types are gone:</p>
  
  <p><img src="https://i.imgur.com/3PEzTte.png" alt="IMG3"></p>
  
  <p>Is there a way to retain columns types? Or should I change types in the end of a query?</p>