I have found a temporary solution for this BUG.
1. Close your game window (must do this before next step) as for Why is it required to do this - multiple game instance is not allowed AND you can't log out of your account right now.
2. Open an incognito tab and then open dirty league site. You can now have fresh log into your account with Auto Mode turned off.
Do not click on Auto mode during your gaming session.
Thanks to this, I was able to finish all goals in colosseum event.
I hope this annoying BUG gets fixed soon. Also it would be better if Auto mode is removed from Colosseum fights.