If an Excel spreadsheet is formatted in a certain way and added to ArcGIS Pro the first row is considered as values and the default headings are used. This happens in a couple of circumstances. The first is if a cell on the first row is null then the whole of the first row are treated as values and not as a headings and the default headings are used. This issue is reported here
This is illustrated below.
In the example above the table on the left is added to Pro and the first row is used to create the column headings. The same table on the right is added to Pro, except it has a cell on the first row that is null, then the default headings are used (Field1, Field2, …).
The second formatting that leads to the first row being treated as values and not headings, is if all of the first value row is text.
The example above shows the table on the left that only has text values. When added to Pro the default headings are used. When the same table has a number on the first value row then the correct headings are used.