Your confusion seems to come from the term “reservation”. Reservation means to save something before the time it is to be used. In this case, it means to save your name before the server becomes active.
As for the reason your name was changed, there has been some trouble recently where players have created toons on the RK2019 server using the names of established toons on the LIVE server and then impersonating the players of said toons. FC has stated that they will assist players in reclaiming their name in some cases if asked. It’s possible that you have inadvertently taken the name of an established LIVE toon, and the player of that toon has requested it.
There also a possibility that your name was offensive and was reported.
If you think this was done in error, you can try submitting a ticket to support and making a case for return of the name. Alternatively, you can at least request your name changed to something different from defaultxxxx.
I think OP is confusing the meaning of reserve. This reserve is intended to be like, you can’t hold on to it before you make the character, but DOES NOT mean that you aren’t entitled to have the name you have already had. This game is pretty old, and many people have had the same name for 10-15 years. To not have that name on the new server would be like a slap in the face.