Substring and Lookup

You can split the comma-separated column into new rows, merge on that column and Group By to recombine.

Here's sample code you can paste into the Advanced Editor of a new blank query to see what I mean:

let
    Lookup = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSjJU0lFyVIrVATKNgEwnCNMYyHRWio0FAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Key = _t, Value = _t]),
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Hci5EQAwCAOwXVy74dmGowD23yGBUoqAgChhC0eQDOiPVo6y9MI2jGUnXzkyHw==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ID = _t, CSV = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"ID", Int64.Type}, {"CSV", type text}}),
    #"Split Column by Delimiter" = Table.ExpandListColumn(Table.TransformColumns(#"Changed Type", {{"CSV", Splitter.SplitTextByDelimiter(",", QuoteStyle.Csv), let itemType = (type nullable text) meta [Serialized.Text = true] in type {itemType}}}), "CSV"),
    #"Merged Queries" = Table.NestedJoin(#"Split Column by Delimiter", {"CSV"}, Lookup, {"Key"}, "Lookup", JoinKind.LeftOuter),
    #"Expanded Lookup" = Table.ExpandTableColumn(#"Merged Queries", "Lookup", {"Value"}, {"Value"}),
    #"Grouped Rows" = Table.Group(#"Expanded Lookup", {"ID"}, {{"CSV", each Text.Combine([CSV], ","), type nullable text}, {"Value", each List.Max([Value]), type nullable text}})
in
    #"Grouped Rows"