Try the following, and see if you think this is strange.

start with an empty workbook with three worksheets.
create a book level defined name "aaa" on Sheet1
create a sheet level defined name "aaa" on Sheet2
Make Sheet1 the activesheet.
In the VBA immediate window do msgbox
range("aaa").address(external:=true)
you should get the range on Sheet1
make Sheet2 the activesheet.
In the VBA immediate window do msgbox
range("aaa").address(external:=true)
you should get the range on Sheet2
make Sheet3 the Activesheet.
In the VBA immediate window do msgbox
range("aaa").address(external:=true)
you should get the range on Sheet1

So far, so good.

Now make a chart and put it on a separate chartsheet.
make the chartsheet the activesheet.
In the VBA immediate window do msgbox
range("aaa").address(external:=true)
I get the range on Sheet2

I think with the chartsheet active, the book level name should be
returned as when Sheet3 was active.

I wonder if this is a bug in VBA?