Field has List values and type ABC123 values.

See this example:

let
    Source = List.Zip({{1..10}, {21..30}}) & {1},
    #"Converted to Table" = Table.FromList(Source, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Added Custom" = 
        Table.AddColumn(
            #"Converted to Table", 
            "Custom", 
            each 
            if Value.Is([Column1], List.Type) 
                then Text.Combine(List.Transform([Column1], Text.From), ",")
            else 
            [Column1]
        )
in
    #"Added Custom"

I added a 2nd column to either expand the list or pull the raw value.

How to use M code provided in a blank query:
1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done
5) See this article if you need help using this M code in your model.