Drupal 8 creates <link rel="canonical" href="/relative/url" /> by default. support.google.com says:
Use absolute paths rather than relative paths with the rel="canonical" link element.
Use this structure: https://www.example.com/dresses/green/greendresss.html Not this structure: /dresses/green/greendress.html
I used the Google Search Console URL Inspection and found Google does not recognize relative urls as canonical. It says:
User-declared canonical N/A
This caused some google indexing issues with our D8 Multi-Language website.
Please see also: https://drupal.stackexchange.com/questions/270150/link-rel-canonical-hre...