Especially in combination with your answer to Papa Echo below, which seems to exclude a cause inside the database (some 'on change' macro on the field, that could be a very unlikely explanation). I'm sure you already tried it directly in the table and via a form to see if that makes any difference.
And it would be a very mysterious virus in your computer that's trained to change one certain name in one certain field in one certain table.
You can learn to live with your work-around of entering the 'wrong' name. Make a small update query to change it back to the right one, and you've got a situation you can handle.
Or you can continue to search. Make a new database, with just one table with just one name field and see how it behaves. Then in the old one export this table to Excel, and import it in the new one, creating in essence the same table (column names and data are copied) and see how that behaves. If still OK, import everything else.
It could well be that after this forced migration everything is fine in the new database. But you will have to recreate all relationships in the relationship window, and all properties (including indexes) of the 'new' table, such as number type and field length (those are not copied). You might even be able to import the whole table into a new database and see that it works correctly.
Or delete the table in the current database (after exporting the data to Excel), reorganize and import it again. This might delete the unknown.
I really don't know. But from your posts, you seem qualified to do some more research and possibly find a better solution than the current work-around.
Good luck,
and please let us know the results.
Kees