Table scan and joining data in a sigle value

Sorry. I see the issue now. I had unnecessary table code in there. That column is being treated as an embedded table and it isn't. New code:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjDUByIjAyNjJR0lZ58gIOkIxAGleSWliTmZKYkpSrE6OJWFJBalJuJTEJRaUpRYnK/gmpValJyZnJmPqdg3yN2XGFuR1OGwFkkFkn2xAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Fecha = _t, Alumno = _t, Materia = _t, Comentario = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Fecha", type date}}),
    #"Grouped Rows" = 
        Table.Group(
            #"Changed Type", 
            {"Fecha", "Alumno", "Materia"}, 
            {
                {"Comments", each Text.Combine(_[Comentario], ", ")
                }
            }
        )
in
    #"Grouped Rows"

Basically you are removing the code below in yellow, including that first comma.