Export usingUnderlying datawill not work if the data source is an Analysis Services live connection
When using DirectQuery, the maximum amount of data that can be exported is 16 MB. This may result in exporting less than the maximum number of rows, especially if there are many columns, data that is difficult to compress, and other factors that increase file size and decrease number of rows exported.
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.
This is odd behavior for sure. It will likely take more interation and back and forth with specifics about your datasource. I think the best course of action would be to engage customer support so they can look at your private information securely.