Sometimes that happens, apparently just a glitch where the last dog goes outside the map, never returns, and since you have to kill it to end the round, the round never ends. But if there is any specific trigger that causes the glitch, I'm not sure.
Yes i know how to make that happen it has happened to me 3 times already and me and my friends even saw the dog run through the map and we tried to kill it but it wouldnt die. but what you have to do is go underground you know what little space where that one window is underneath the quick revive and get in that dog glitch thats in the corner then when you have a dog jumping in front of your face DONT kill it let another one come line up behind it and then another one but that last one should run out the window i have done this once a long ass time ago i dont know if it still works but thats what i did to get it to do that and the dog will return to that window every 10 minutes i think its in a glitch where it runs around the map but yeah every 10 mintues it will run through that window but we have never been able to kill it even with bettys so dont try this if you still want to contuine because i dont think theres anyway.