I guess you are doing it wrong... See the StdActionBar Example-Code!
B4X:
bar.Initialize("bar")
bar.NavigationMode = bar.NAVIGATION_MODE_TABS 'change to list to see the other mode
bar.SetListItems(Array As String("Dropdown 1", "Dropdown 2", "Dropdown 3"))
bar.Subtitle = "This is the subtitle"
bar.ShowUpIndicator = True
Activity.Title = "This is the title"
It's not a bug, it's the order you are performing the instructions. Each layout file has an activity title attribute which will set the title when you load it. So, in your example:
B4X:
Activity.Title = "aaaa" ' <=== Sets the title to "aaaa" at this instant -- display will not change as the sub is still running, blocking UI updates
And then,
B4X:
Activity.LoadLayout("layoutmain.bal") ' <==== Now sets the activity title to whatever you defined in the layout
If you want, try moving your activity title change to the last line of your Activity_Create and see if it does what you wish.