Finally I was able to detect the problem. TabControl was templated with a bunch of different controls, while AutomationPeer stayed the same which is only aware of TabItems as TabControl children. I subclassed TabControl and overrode OnCreateAutomation to create and return my GenericAutomationPeer, which can enumerate all child UIElements of this control and voila - UIA Verify can now see that additional controls from TabControl template.
GenericAutomationPeer implementation was found here: http://www.colinsalmcorner.com/2011/11/genericautomationpeer-helping-coded-ui.html Great thanks to the author of that article!