JSON - Expand Multiple Arrays to Columns

GregLukas , you might want to try,

let
    Source = Json.Document("{
  ""items"": [
    {
      ""id"": ""E1"",
      ""stat"": [
          {
            ""columnA"": [
              ""E1valA1"",
              ""E1valA2"",
              ""E1valA3"",
              ""E1valA4""
            ],
            ""columnB"": [
              ""E1valB1"",
              ""E1valB2"",
              ""E1valB3"",
              ""E1valB4""
            ]
          }
      ]
    },
    {
      ""id"": ""E2"",
      ""stat"": [
          {
            ""columnA"": [
              ""E2valA1"",
              ""E2valA2"",
              ""E2valA3"",
              ""E2valA4""
            ],
            ""columnB"": [
              ""E2valB1"",
              ""E2valB2"",
              ""E2valB3"",
              ""E2valB4""
            ]
          }
      ]
    }
  ]
}"),
    items = Source[items],
    #"Converted to Table" = Table.FromList(items, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Expanded Column1" = Table.ExpandRecordColumn(#"Converted to Table", "Column1", {"id", "stat"}, {"id", "stat"}),
    #"Expanded stat" = Table.ExpandListColumn(#"Expanded Column1", "stat"),

//major step of the solution
    Custom1 = Table.TransformColumns(#"Expanded stat", {"stat", each Table.FromColumns(Record.ToList(_))}),

    #"Expanded stat1" = Table.ExpandTableColumn(Custom1, "stat", {"Column1", "Column2"}, {"Column1", "Column2"})
in
    #"Expanded stat1"