If my memory serves me well, I think there is currently a limitation as to how many people can join mid-round. If not, then I'll talk to Ross and the rest of the Lead Team and think about how this system should work. We could also do something that Josh did on Murder, and that would be to wait an X amount of time (60 seconds on Murder) upon map change which gives all players who were playing before, sufficient and adequate time to load in - whilst also completely disabling mid-round joining. The timers in-between rounds would not be changed though.
I'll discuss this with the TTT LT and see what we're gonna do about it. Expect a change in the ensuing patch/update. Thank you for the suggestion!