6 queries in DataFlows returning "Key Didn't Match Any Rows in Table" error...

I'm at a loss as to what the issue could be, so I'm turning to this community with the hopes that someone can help me.

I've inherited the support/management responsibility on a dataflow and 2 PBI reports that were created by a third party vendor months before I joined this organization. Up until today, I was able to resolve issues, but not this time. 

I'll do my best to describe the set up for you all, as the DataFlow is fairly complex.

Data Sources: All the data is stored and updated in Excel sheets in SharePoint. Every quarter a new folder and file is created in a specific directory, and then updated as required.

Within those files, are 2 Excel pages with properly labelled tables. They are all locked, so even I can't go in and change the column names, or table names. The 6 queries in question refer to 6 tables in page 2 of each Excel sheet. But they don't refer to them directly.

There's 1 query that acts as the source for those 6 queries. And that source query pulls data from 6 Custom Functions. This is where I reach the limit of my knowledge, since I have no experience using custom functions in Power Query. But from what I gather, those functions grab their respective tables from each of the Excel files. So it looks something like this:

Excel Tables -> 6 Custom Functions -> Source Query -> 6 Queries.

Each of the 6 queries removes all but the relevant columns to that query. The Applied Steps in each query look like this:

And each query fails on the third step, "Expanded...". Right after the specific columns are selected, the expansion fails and this error pops up:

I checked the Excel tables, and all of the tables are there. They're all labelled correctly. The columns that I'm trying to expand are all there as well. I can't figure out what the issue is.

If I need to provide more info, I'll do my best. 

Thank you