I'm experiencing somehow the same problem, but I have applied your suggestions and it doesn't work.
Here is the story:
Due to some project/stability related reasons we use version R3 2018 – 188.8.131.522.
I'm using the Designer integrated in VS 2017 Enterprise.
The report was built by another colleague and it was working fine. This uses textboxes and a table where all the fields are mapped. Now I had to add an additional field/column to it. I have done so (only the column without any text) by adding an extra column (in the middle) of the table and I have resized the Details section to fit or even be wider than the table.
The issue is that I still get an extra column when exporting the report to excel. This extra column is merged with the next one for displaying one field of the table.
Please see the screenshots attached.
As a remark, I have applied your fix with success in the past on the same report.