Such issue could occur due to wrongly cached simulator files. If this is the case, you will need to navigate to the %localappdata%\Telerik\BlackDragon\Cache folder and delete all of its content. Note that, all AppBuilder clients must be turned off before that. After you have deleted the cache, please check if the issue still remains.
Also, I must note that with the yesterdays AppBuilder 2.5 release we introduced a critical bug within the simulator that prevents the correct rendering of CSS transitions. The good news however is, we have scheduled a hotfix for this for next week.
Until then, you should be able to test your applications in the In-browser client, actual devices or in the native emulators.
I apologize for any inconveniences caused by this and thank you for the understanding.