Back to the advice of providing clear and consistent signals to Google - Google's John Mueller had to tell someone on Twitter that Google won't always recognize rel=canonical as the ultimate decision on which pages should be the canonical page. There can be other signals that confuse Google and lead to Google making a different decision.
Google's John Mueller said "we use multiple factors when determining the canonical for a page, a rel=canonical isn't a guarantee, so ultimately that can happen." Then he pointed out a confusing signal he saw: He said "rel=canonical to the first page + rel next/prev are a bit conflicting, either it's the same as the first page, or they're paginated series."
We use multiple factors when determining the canonical for a page, a rel=canonical isn't a guarantee, so ultimately that can happen. Also, rel=canonical to the first page + rel next/prev are a bit conflicting, either it's the same as the first page, or they're paginated series.
— John ☆.o(≧▽≦)o.☆ (@JohnMu) July 20, 2018
This is shortly after covering how noindex and rel canonical can cause confusion as well.
Again, be clear and consistent with your signals to Google.
Forum discussion at Twitter.
