Custom function works until I try to invoke it on a column of nested tables.

Background:

I'm trying to read hundreds of PDF pages of accounting general ledger detail reports into a clean data table. Power query's ability to parse this data is suboptimal. It seems to read the PDF from left to right, so report column names that are left-aligned end up with their own column while the values that are right-aligned end up in the next colum. A Purchase Order ID column header might line up with its data of five-digit values, but but then a manually generated two-digit PO gets thrown into its own column. The result is that my data gets stretched across an execssive number of columns inconsistently from one page to the next and makes it seemingly impossible to buid a transform query that will address each page of the PDF.

My Approach:

I wanted to create a function that would dynamically merge the columns of each PDF page under the report's data headers to force a consistent table structure from one page to the next. The plan was to build a nested function that would accept a PDF page, dynamically identify the default columns to merge for each data header, call a function to iterate through each row of data header and columns list calling a function to merge columns.

Result:

I can invoke my function on a query of any page of the PDF, but when I try to invoke it on the table where each page resides as a nested table within the [Data] column, 100% of my results are errors, even on the pages I've individually invoked successfully. I've been using ChatGPT to troubleshoot, it seems like my issue is "lazy evaluation" inside a row-level operation, but I haven't had any success resolving this.

Request:

Does anyone know how to diagnose or overcome this "lazy evaluation" that appears to be gumming up the works? Are there any best practices or common patterns that might be more effective when dealing with PDF files?

(SourcePage as table) as table =>
let
    // Define the function to merge a single set of columns
    DynamicColumnMerge = (SourceTable as table, MergeList as list, ColumnName as text) as table =>
        let
            MergeColumn = Table.CombineColumns(
                SourceTable, 
                MergeList, 
                Combiner.CombineTextByDelimiter("", QuoteStyle.None), 
                ColumnName
            )
        in
            MergeColumn,

    // Define the function to iterate over multiple merge instructions
    IterativeColumnMerge = (SourceTable as table, MergeParams as table) as table =>
        let
            MergedTable = List.Accumulate(
                Table.ToRecords(MergeParams), 
                SourceTable, 
                (currentTable, row) => DynamicColumnMerge(currentTable, row[MergeColumns], row[NewColumnName])
            )
        in
            MergedTable,

    // Main query logic
    Source = SourcePage,
    Indexed = Table.AddIndexColumn(Source, "Index", 0, 1, Int64.Type),
    #"Added HeaderRow Column" = Table.AddColumn(Indexed, "HeaderRow", each 
        if [Column1] = "GL Date" then [Index] 
        else if [Column2] = "GL Date" then [Index] 
        else if [Column3] = "GL Date" then [Index] 
        else null
    ),
    #"Filtered HeaderRow" = Table.SelectRows(#"Added HeaderRow Column", each ([HeaderRow] = 3)),
    HeaderRow = #"Filtered HeaderRow"{0}[HeaderRow],
    #"Jumped Back to Source" = Source,
    #"Removed Top Rows" = Table.Skip(#"Jumped Back to Source", HeaderRow),
    #"Kept First Row" = Table.FirstN(#"Removed Top Rows", 1),
    #"Demoted Headers" = Table.DemoteHeaders(#"Kept First Row"),
    #"Transposed Table" = Table.Transpose(#"Demoted Headers"),
    #"Filled Down" = Table.FillDown(#"Transposed Table", {"Column2"}),
    //MergeLists = Table.Group(#"Filled Down", {"Column2"}, {{"Count", each _[Column1]}}),
    MergeLists = Table.Group(
        #"Filled Down", 
        {"Column2"}, 
        {
            {"MergeColumns", each _[Column1], type list},  // Ensure this is a list
            {"NewColumnName", each Text.Combine(List.Distinct(_[Column2]), "_"), type text}  // Generate a column name
        }
    ),


    // Apply the iterative merging function
    FinalMergedTable = IterativeColumnMerge(Source, MergeLists)

in
    FinalMergedTable