It does sound strange but I'm not yet set on it being a corrupted file.
Was this document working correctly before (i.e., you have used it before today and it was working the way you expect) and this time when you opened it it is acting like this?
Does the new row actually delete itself from the table? The row number is gone and the remaining rows are all sequentially numbered with no gaps in the numbers? When you try again and add a new row, is it also sequential with no gaps in row numbers? I am trying to determine if it is getting hidden/filtered vs getting deleted.
You said "[when] I select a new Merchant". Is the Merchant column a pop up menu? I know you said "automatic" or "text" but you used "I enter" for the other columns and "I select" for this one. The verb "select" implies a selection, like with a pop up menu. Pop up menus will automatically fill to the next row.
Is there a filter on the "description" column, now that you said this is the column that makes the row disappear?
What do you mean by "everything I have entered on that new row disappears and then that new row I am entering data on disappears"? Is this a two step thing? At what point does "everything on the row disappear" and at what point does "the new row disappear"?