The HTTPS migration is very delicate for SEO: if done wrong, it can be extremely negative. This is because Google treats the two versions (HTTP and HTTPS) as two completely different websites; having both versions active and working, without redirecting to the HTTPS version, means duplicating the website, which will cause issues to your ranking.
Furthermore, although Google stated that the HTTPS version has a higher consideration in its algorithm, sometimes the migration causes a temporary drop in ranking (from 1 day to 2 weeks, or more). This is due to the redirects from HTTP to HTTPS.
Once Google “understands” the migration, the ranking will come back, and often be better than before!
If you’re worried, don’t be: it’s in Google’s interests that all websites migrate to HTTPS without issues; you just have to follow a few simple rules and wait for Google to recrawl the website.