Most publishers define the order of the a b as how it will be listed in the bibliography, and not the order it is cited in the document. So b coming before a is not uncommon. There are tricks to overcome it, but is it really necessary?
If you want to, you can check this thread with links to various treads about this kind of request. https://community.endnote.com/t5/EndNote-General/Citation-sort-order-for-same-year/m-p/154377/highlight/true#M32448