I think it has something to do with a topic a staff member here posted a while back showing a change they wanted to make with how creators were displayed. It involved badges and a shorter view so that you don't have to look at the whole profile (maybe a new browsing mode IDK)
But in that image it showed 'author of ___ and 4 others' so it could be a sign that they are in fact rolling out that new creator view, and the main series button is to designate what is named and what is an 'other'
No changes that I've noticed YET, but big code changes take a while to roll out so we just have to be patient ^-^