The same issue is also reported here. It looks like the discount being applied is not 10% as the description states.
I can't reproduce any pricing issue while logged in with my LN discount, so it might not be affecting every account. (The distinction probably is because I also own the feature set, and can't opt in or out of buying one with a library.)
As an aside a similar issue existed very briefly when they first started the Summer Sale, but it was very quickly corrected. Even though I owned a feature set, I was offered the option to "not" buy one and get an additional discount, which obviously wasn't correct. I'm sure this issue will get quickly fixed.
Just out of interest, have you tried adding the Extended Feature set after adding a Library to your basket to see if that triggered the extra Library discount (assuming you only have the Full Feature set)? https://www.logos.com/product/136012/logos-7-extended-feature-set
The extended feature set is the one I own, so I can't do that. I imagine though that anyone who doesn't own a feature set could reproduce that bug.
It even seems to trigger the additional discount if you use the Verbum starter set with a Logos Library https://www.logos.com/product/121389/verbum-7-starter-feature-set