I believe there is an issue with the Ext.Msg.alert() dialog in chrome right from the first version of the Sencha Touch. However there were css hacks that have been posted in various forums to handle them.
My point is it only has issues with Chrome and not with safari and I believe mobile devices dont use chrome yet. So I just had this tought running in my head.
Just leave it and get on with your work unless u think, it will keep bothering you. :)