Nope, not a bug to loose connection to host when an arena is full.
That is as it has been, so I am just guessing here that you have never tried to enter a full aren prior to this.
Sometimes it will even do it with a few less than the full number, due to guys who have entered between the time the numbers were sent to your computer and the time you ring the doorbell, and, conversely, the reverse is true. Occasionally, you can click a full arena and it well let you in because someone has left in the same time frame as described above.