Unfortunately, with the yesterdays AppBuilder 2.5 release we introduced a critical bug in our simulator. It prevents the correct rendering of CSS transitions.
An urgent AppBuilder hotfix is now scheduled for next week, that should bring the simulator functionality back to normal. Meanwhile, you can check if clearing the simulator's cache improves its behavior. For that purpose, you will need to delete the whole content of the %localappdata%\Telerik\BlackDragon\Cache
folder after turning off all AppBuilder clients.
Further, if the above does not fix the matter, I would suggest testing your applications inside the AppBuilder In-Browser client, actual devices or native emulators until the fix is present.
Please, accept our apologies for all inconvenience caused and thank you for the understanding.