"I wonder if it's because the whole Navigation Bar is treated as one object"
That's very likely the issue here...
"and how do I get around it"
Well...
I want to click on the tab called "Header" header.png
I name map the tabs line namemapping.png
Now to find the tab
Use ObjectSpy on the tabs line objectspy1.png
Switch to Advanced mode to get more properties objectspy2.png
Scroll down through the list and look for something that might be a collection of objects. In my case, VisibleTabs looked like a good choice. Click on the button on the right with the three dots to expand the property. objectspy3.png
Look for something with a Params button, in this case, Index, and click on the Params button objectspy4.png
Index may have a number or a text entry or both. In this case, we need an integer. "Header" is the first tab, so index for this one is 0. Enter that in the box and you will see it on the property. objectspy5.png
Next click the three dots button on the right and see what you can find out about Index(0). The text property = Header, so we're almost done. objectspy6.png
Drill down on the three dots again and find OleValue property that says Header. That's the actual text inside the object. Click on that. objectspy7.png
Now you'll find a long definition up in ObjectSpy that ends in OleValue. Copy it and save it in notepad or somewhere for later reference.
In my test example, I used:
If (object property BigLongOleValue) = "Header" Then
OnScreenAction (object we mapped at top) ClickTab index = 0
test.png
and magically it clicks the tab!