Trouble adding anchor by reference

The picture below should explain my following conundrum. I was wanting to add an anchor for Josephus' War of the Jews 6.300-309. I eventually managed to do this by selecting the entire text in Josephus and adding an anchor by "selected text". That is the first anchor at the top left.

If I select 6.299 in the reference box of Josephus, I can add an anchor by "Active Reference". I assumed then that if Logos recognises the specific reference of "War of the Jews 6.299", then it should recognise "War of the Jews 6.300-309". Except that it doesn't. (I can, of course, get by with what I did in the first paragraph; however, it would make it simpler if I could just have the anchor as a milestone.)

You can see from the picture that the drop down menu of options does not even give me the option for "War of the Jews" - even if I don't add the specific reference.

I don't understand why I can add an anchor by "active reference" to a specific milestone but can't type in the same milestone and add an anchor...