Power BI Cannot find a Column in a Replacement File

Using four .csv files, I created a From File From Folder model in PBI Desktop and it worked as expected.

Then I realised one of the files contained the wrong day's data so I simply updated the file in the source folder, as I have done so many times before. When I refreshed the Query, however, PBI told me it could not find the Timestamp column (see below) in the files: in fact, it was in the .csv files and it was the only column I removed from my PBI model.

I clicked to look at the error and tried to tweak the M code to ask PBI not to look for the Timestamp column; but then it told me there was another column it could not find! I checked the source files but could see nothing wrong with the replacement file.

My original columns and what I did to them is as follows:

Timestamp ... removed

UTC ... split into two columns, Date and Time

Callsign ... left alone

Position ... split into two, Latitude and Longitude

Altitude ... left alone

Speed ... left alone

Direction ... left alone

Because of this, I had to start from scratch and I am sure I shouldn't need to do that!