Power Query: Working Query Fails if Moved Out of Workbook into External File

Question

I am using Power Query in Excel for Microsoft 365.

This post is a follow-on to this one:
Power Query: Combine Subset of List of Tables that Exist

The Power Query shown below is not the solution I ultimately went with in the post linked above, but it does present a problem I am trying to understand.

Consider this working Power Query (which is saved directly in my workbook):

// Combine the subset of tables Table1, Table2, Table3 that actually exist.
let
    remove_errors_f = (input_list as list) as list =>
                          List.RemoveNulls(
                                              List.Transform(
                                                                List.Positions(input_list),
                                                                each try input_list{_} otherwise null
                                                            )
                                          ),

    combiner_f = (AccumulatedTable as table, TablesToCombine as list) as table =>
                 if List.Count(TablesToCombine) = 0 then
                     AccumulatedTable
                 else
                     @combiner_f(Table.Combine({AccumulatedTable, TablesToCombine{0}}), List.RemoveFirstN(TablesToCombine, 1)),

    #"MyTables" = remove_errors_f({Table1, Table2, Table3}),
    #"All Column Names" = {"Column1", "Column2", "Column3"},

    Source = combiner_f(#table(#"All Column Names", {}), #"MyTables"),
    #"Source Sorted" = Table.Sort(Source, List.Transform(Table.ColumnNames(Source), each {_, Order.Ascending}))
in
    #"Source Sorted"

Now, suppose I move this Power Query out of my .xlsx file and move it character-for-character verbatim into a text file named combine_worksheet_subset.pq. (This is for the purpose of version control.)

I then create a "stub" Power Query directly in my workbook to load and execute combine_worksheet_subset.pq:

let
    // The source must be an absolute path
    Source = Text.FromBinary(File.Contents("C:\my_path\combine_worksheet_subset.pq")),
    EvaluatedExpression = Expression.Evaluate(Source, #shared)
in
    EvaluatedExpression

This works fine as long as all of Table1, Table2, and Table3 exist. However, it fails if any of these tables are not present. For example, if I remove Table2, I get the following error:

Why does this Power Query have this failure case when it is loaded from an external file (but not if it's stored directly in the workbook)?