Looks like we can't reproduce the issue or there's a problem in the test case provided.
Problem with tabbar when panel scroll in Tabs2 example
Tabs2 example from ST2 PR2,PR3 is not working properly on my Android. When I scroll panel the icons on tabbar become the blue boxes. I test on Desire HD with Android 2.3.5, WebKit 533.1. Same problem is on Kindle Fire. PC Safari and Chrome is OK. See the picture below.
Sencha - Sencha Touch Dev Team
This is because when you use CSS 3D transform (on those devices), webkit icon mask no longer works. Strange, eh?
I'm not sure if we will have a fix for this before the GA, but we will try hard. If you really want to get around it in your application, you'd have to not use iconMask.