Yes, NT players can only be sold, not fired. If they are put on the transfer list automatically (team banned, inactive) they are put at their salary*10 price, max 1M. If they are not sold, they retire just like other players. It means nobody was willing to spend 1M on him and pay for his salary after that. Nothing wrong there.
Also, during the offseason, we will clean up all the NT's from retired players. Also, we will add code that removes a player from a NT once retired.