The problem came up again when I updated my theme.
I tried switching themes again to see if it worked with my other themes but it did NOT resolve the issue. Turning off my plugins did not help either so nothing is overwriting it.
I really like this plugin but it just does not work.
I think I resolved the issue.
I activated a different theme and suddenly the series showed up on the list again.
My current solution was recreating my child theme and then activating it.
