Exposed the HandleKeyDown method of RadAutoCompleteBox. In order to navigate to the next control when Tab key is pressed, the method should be overridden as shown below:

public class CustomAutoCompleteBox : RadAutoCompleteBox
{
protected override bool HandleKeyDown(Key systemKey)
{
return base.HandleKeyDown(systemKey) && systemKey != Key.Tab;
}
} 
Available in LIB version 2016.2.525, it will be also available in the 2016 R2 SP1 release.