I have never seen this. Happens on one specific field on one record.
Last Name is 'Than'.
Want to change it to 'Tahn'.
I place the cursor after the 'T' and add an 'a' and move after the 'h' and delete the 'a'.
When I tab to the next field the data switched back to original value.
However, if I merely add characters to the end of the name it takes this update fine.
You may have some spell check process running on this machine or something like that. there is something called T9 english which makes words depending on the letter so when you type 'Tahn', it think you meant "Than" and so changes it accordingly. I don't think Access is doing anything here.