Incorrect datas change

Some time ago I published a youTube video presenting FMRTE and what user can do with (https://www.youtube.com/watch?v=KcnfpdvWKdc), and someone asked me if it was possible for him to create a player in his own team and have him to be seen as his son. I answered that, strictly speaking, he couldn't create a new player and that it was the role of the SIGames editor before creating a save, but that he could modify one in its totality.
As I'd never had this idea before, I wanted to try modifying a player by myself in my own save (surname, first name, height, weight, date of birth and nationality, just for test purpose) and then I came across a real problem, in my opinion a FMRTE bug.
Each player is identified by a unique identifier, but you can only change the first and last name of the player if you preserve BOTH the length of the last name AND the length of the first name. Why can't I replace Tom GUPPY with (for example) Thomas ROBERTS ? (Doing that give as result : Tho ROPER 3 letters for the firstname and 5 for the lastname)
What's more, the warning message gives incorrect information, suggesting that I were going to modify several records at once (see attached screenshots).
Thanks to correct it and letting me know.