Google can put a full year to index your site on HTTPS after the transfer. It was my experience. For more details, see my response to HTTP to HTTPS: Do you expect the new sitemap to be indexed?
During this first year, Google can still index the HTTP version of some pages stating that they "were choosing another holder than the canonical." Month after month, you'll see more and more pages going to HTTPS in the Google Search Console.
This does not seem to bother the rankings, it just means that some users are still sent to the HTTP version from Google search for a while. Since you are redirecting, they finally arrive at the correct HTTPS version.
I was really surprised when I saw that Google was so slow to completely transfer my sites to HTTPS. My theories about Why:
- It takes time to build enough trust in your HTTPS version.
- Each URL must go through some sort of verification process in which it compares the content of the old URL to the content of the new URL. Since the slowest URLs to move are the least popular with the least ranking, it is possible that they give priority to the most popular pages of the process.