Thanks for your patience.
Query 1: CenterButtonBackView shows badly
In android we could able to reproduce the reported issue of “ CenterButtonBackView shows badly” and currently we are validating the same and we will update the complete details on January 23,2020.
Query 2: In general, it shows different on android and IOS
We have checked your provided sample and you can avoid the alignment and Item hidden issue in iOS throughout the sample level. SegmentIndex will work only for LayoutType as Custom, in your provided sample you have not set LayoutType, by default it will take LayoutType as Default. By removing the SegmentIndex you can avoid the Item hidden issue in iOS.
We have modified the sample based on your requirement please find the sample from below location.
Sample : https://www.syncfusion.com/downloads/support/directtrac/general/ze/RadialMenu-2003562600.zip